iOS port M0b: extract Transport + ServerContext to ScarfCore

Second of four M0 sub-PRs. Moves the remaining cross-cutting
infrastructure — the ServerTransport protocol and its two implementations
(LocalTransport, SSHTransport), plus ServerContext and its helpers —
into ScarfCore so both Mac and (future) iOS targets share one codebase.

Files moved (5):
  - scarf/Core/Transport/ServerTransport.swift (+ FileStat, ProcessResult, WatchEvent)
  - scarf/Core/Transport/LocalTransport.swift
  - scarf/Core/Transport/SSHTransport.swift
  - scarf/Core/Transport/TransportErrors.swift
  - scarf/Core/Models/ServerContext.swift (+ SSHConfig, ServerKind, ServerID, UserHomeCache)

Split out of ServerContext.swift into a new Mac-target sibling file
scarf/Core/Models/ServerContext+Mac.swift:
  - runHermes(_:timeout:stdin:) — depends on HermesFileService
  - openInLocalEditor(_:) — depends on AppKit.NSWorkspace
These methods can't live in ScarfCore itself because ScarfCore must not
depend on main-target services or AppKit. iOS will provide a sibling
ServerContext+iOS.swift in M2+.

Removed: scarf/Core/Models/HermesPaths+Deprecated.swift.
  Zero callers in-tree; its only justification was that ServerContext
  used to be in the Mac target. With ServerContext in ScarfCore now,
  the deprecated forwarders are both unreachable AND dead code.

Breaking the ScarfCore → main-target circular dep in SSHTransport:
  The old SSHTransport.sshSubprocessEnvironment() called
  HermesFileService.enrichedEnvironment() to harvest SSH_AUTH_SOCK from
  the user's login shell. Replaced with a local #if os(macOS) helper
  SSHTransport.macLoginShellSSHAgent() that probes /bin/zsh for only
  the two SSH agent vars (no PATH/credentials — that's still in
  HermesFileService for ACP subprocess use). Behavior-identical on
  macOS; no-op on iOS/Linux.

Platform guards added in ScarfCore (runtime targets still macOS/iOS):
  - `#if canImport(os)` around os.Logger (definition + every call site,
    except the large Darwin-dependent ensureControlDir block).
  - `#if canImport(Darwin)` around LocalTransport.watchPaths (FSEvents)
    and SSHTransport.ensureControlDir (Darwin.stat/lstat). Linux gets
    a no-op empty stream and a best-effort FileManager.createDirectory
    fallback — neither is exercised at runtime on Linux, only compiled.
  - `#if canImport(SwiftUI)` around ServerContext's EnvironmentKey.
  - `#if canImport(AppKit)` inside the new ServerContext+Mac.swift
    extension.

Bug fixed: M0a's sed transform accidentally added `public` to protocol
requirements in ServerTransport.swift, e.g. `public nonisolated var
contextID: ServerID { get }`. Swift forbids access modifiers on
protocol requirements — stripped.

54 additional consumer files in the Mac target gained `import ScarfCore`.

Test coverage: 18 new tests in ScarfCoreTests/M0bTransportTests.swift.
Runs on Linux via
  docker run --rm -v $PWD/scarf/Packages/ScarfCore:/work -w /work swift:6.0 swift test
Total suite: 34 / 34 passing (M0a's 16 + M0b's 18).

Updated scarf/docs/IOS_PORT_PLAN.md progress log with the shipped M0b
state and the Platform-guard patterns future phases should reuse.

https://claude.ai/code/session_019yMRP6mwZWfzVrPTqevx2y

scarf/docs/IOS_PORT_PLAN.md

@@ -224,7 +224,85 @@ $PWD/scarf/Packages/ScarfCore:/work -w /work swift:6.0 swift test`.
   `nonisolated` to new ScarfCore APIs pre-emptively; match the
   surrounding conventions.
 
-### M0b — pending
+### M0b — shipped
+
+**Shipped:**
+
+- 4 Transport files moved to `Packages/ScarfCore/Sources/ScarfCore/Transport/`:
+  `ServerTransport.swift`, `LocalTransport.swift`, `SSHTransport.swift`,
+  `TransportErrors.swift`.
+- `ServerContext.swift` moved to `Packages/ScarfCore/Sources/ScarfCore/Models/`.
+  The `runHermes(_:timeout:stdin:)` and `openInLocalEditor(_:)` extension
+  methods — the only two that depend on main-target `HermesFileService` or
+  on AppKit's `NSWorkspace` — are split out into a new main-target file
+  `scarf/Core/Models/ServerContext+Mac.swift`.
+- `HermesFileService.enrichedEnvironment()` reference inside
+  `SSHTransport.sshSubprocessEnvironment()` replaced with a local
+  `#if os(macOS)` helper `macLoginShellSSHAgent()` that does a narrow
+  `zsh -l -c` probe for only `SSH_AUTH_SOCK` / `SSH_AGENT_PID` (instead
+  of the broader PATH + credentials harvest that still lives in
+  `HermesFileService`). This breaks the Mac-target dependency from
+  ScarfCore. Behavior-identical on macOS; a no-op on iOS (where the SSH
+  agent comes from Citadel in M4, not the user's shell) and on Linux CI.
+- `HermesPaths+Deprecated.swift` deleted. Its only justification was that
+  `ServerContext` was in the Mac target; with `ServerContext` in ScarfCore
+  now, the deprecated forwarders are both unreachable AND unused (zero
+  callers). Good riddance.
+- Added `import ScarfCore` to 54 more consumer files that reference
+  Transport types or `ServerContext` but weren't already importing
+  ScarfCore from M0a. `scarfTests/scarfTests.swift` also gets the import
+  — its `ControlPathTests` now hits the public `SSHTransport` via
+  ScarfCore.
+
+**Platform guards applied in ScarfCore:**
+
+- `#if canImport(os)` — Apple's `os.Logger` (`import os` + every call
+  site). Linux gets silent logging. **Exception:** the large block in
+  `SSHTransport.ensureControlDir()` uses `Darwin.stat` / `lstat` / `mkdir`
+  / `chmod` alongside its Logger calls — the whole method body is wrapped
+  in `#if canImport(Darwin)` with a simple `FileManager.createDirectory`
+  fallback for Linux (stubbed because SSH isn't exercised at runtime on
+  Linux anyway).
+- `#if canImport(Darwin)` — `Darwin.open`/`Darwin.close` + FSEvents-based
+  `DispatchSourceFileSystemObject` in `LocalTransport.watchPaths`. Linux
+  gets a no-op empty stream.
+- `#if canImport(SwiftUI)` — `EnvironmentKey` / `EnvironmentValues`
+  plumbing in `ServerContext.swift`.
+- `#if canImport(AppKit)` — only in the split-out
+  `ServerContext+Mac.swift`, where `NSWorkspace.shared.open` lives. iOS
+  will provide its own equivalent (`UIApplication.open(_:)`) when the
+  target lands in M2.
+
+**Bug fixed while moving:** the sed transform in M0a accidentally promoted
+`protocol ServerTransport` requirements to `public nonisolated var contextID ...`.
+Protocol requirements inherit the protocol's access level and **must
+not** carry an explicit modifier — that's a Swift compile error. Fixed
+in this PR's ServerTransport.swift.
+
+**Test coverage (`M0bTransportTests`):** 18 new tests that construct
+`SSHConfig` with and without defaults, round-trip it through Codable,
+verify `ServerKind` pattern-matching, pin `ServerContext.local`'s
+hard-coded UUID, assert local-vs-remote path derivation, verify
+`makeTransport()` dispatches to the right impl, exercise `FileStat` /
+`ProcessResult` / `WatchEvent` / `TransportError` shapes + error-classifier
+stderr patterns, and round-trip an actual local file through
+`LocalTransport` (write → read → stat → remove).
+
+**Rules next phases can rely on:**
+
+- `ServerContext` is the canonical multi-server entry point. Any new
+  service added in M0c or later takes a `ServerContext` in its init.
+- `ServerContext+Mac.swift` is the pattern for Mac-only methods on
+  ScarfCore types. iOS will have a sibling `ServerContext+iOS.swift`
+  when the iOS target lands. Keep platform-specific methods out of
+  ScarfCore itself and in these sibling files.
+- Logger pattern: `#if canImport(os) ... #endif` around each call site.
+  If there are 3+ sites in one method, consider wrapping the whole method
+  body in `#if canImport(Darwin)` with a Linux-safe fallback.
+- SSH env enrichment is now self-contained in `SSHTransport.swift`. When
+  iOS's Citadel-based transport lands (M4), it will provide its own env
+  story — the existing macOS helper stays untouched.
+
 ### M0c — pending
 ### M0d — pending
 ### M1 — pending