M0a fixup: unignore local Packages/, add missing files, make Linux CI pass

The initial M0a commit was incomplete: .gitignore's `Packages/` rule
(meant for the legacy pre-Xcode-14 SwiftPM checkout dir) silently
swallowed three new files that SHOULD have been committed:

  - scarf/Packages/ScarfCore/Package.swift
  - scarf/Packages/ScarfCore/Sources/ScarfCore/Models/HermesConstants.swift
  - scarf/Packages/ScarfCore/Tests/ScarfCoreTests/ScarfCoreSmokeTests.swift

The 12 moved models slipped through because `git mv` preserves tracking
across gitignored destinations, but new files in that tree did not.

Fix: add `!scarf/Packages/` override so our local SPM package is always
tracked; keep the top-level `Packages/` ignore for the historical case.

Also verified M0a builds + tests green on Linux via
`docker run --rm -v $PWD/scarf/Packages/ScarfCore:/work -w /work swift:6.0 swift test`.
To make that work, two small, Apple-platform-preserving guards:

  - `sqliteTransient` in HermesConstants.swift wrapped in
    `#if canImport(SQLite3)` — SQLite3 is not a system module on Linux
    swift-corelibs-foundation. Apple builds compile unchanged.
  - `ToolKind.displayName` and `MCPTransport.displayName` wrapped in
    `#if canImport(Darwin)` — `LocalizedStringResource` is Apple-only.
    Apple builds compile unchanged.

Additionally:

  - Package.swift pinned to Swift 5 language mode, matching the Mac app's
    `SWIFT_VERSION = 5.0`. Two types (`ACPEvent.availableCommands` and
    `ACPToolCallEvent.rawInput`) claim `Sendable` while carrying
    `[String: Any]` — strict Swift 6 rejects that. Comment in Package.swift
    flags this for a future typed-payloads cleanup + bump to `.v6`.
  - ScarfCoreSmokeTests now contains 16 tests exercising every M0a
    `public init` so parameter drift fails CI instead of a reviewer.
  - IOS_PORT_PLAN.md updated with what actually shipped, the Linux-CI
    guards + patterns future phases should reuse, and the Sendable
    follow-up flagged under "Rules next phases can rely on".

Test results (Linux, Swift 6.0.3):
  Suite M0aPublicInitTests: 15 tests passed
  Suite ScarfCoreSmokeTests: 1 test passed
  Total: 16 / 16 passed

https://claude.ai/code/session_019yMRP6mwZWfzVrPTqevx2y

.gitignore

@@ -21,7 +21,12 @@ xcuserdata/
 
 # Swift Package Manager
 .build/
+# `Packages/` is the historical SwiftPM checkout dir for downloaded deps
+# (pre-Xcode-14). We keep it ignored — but NOT our local-package checkout
+# at scarf/Packages/, which is part of the source tree (ScarfCore, etc.)
+# and must ship in the repo.
 Packages/
+!scarf/Packages/
 Package.pins
 Package.resolved
 *.xcodeproj/project.xcworkspace/xcshareddata/swiftpm/