Migrate GitService.run from Process to swift-subprocess

[poolcamacho]

Context

GitService.run and its helpers (makeProcess, attachDevNullStdin, waitForProcess, startDraining, awaitDrain) hand-roll Foundation's Process API. This has cost us a series of bugs that are non-issues with a higher-level abstraction:

• NSPOSIXErrorDomain code=9 / EBADF from posix_spawn when reusing the shared FileHandle.nullDevice for stdin
• The pipe-buffer deadlock fixed in fix(run): start pipe drain before waitForProcess to avoid 64KB deadlock #6 (drain tasks must run for the whole process lifetime, not after the wait)
• Manual timeout polling that ties up a thread on a global queue (already tracked in Replace polling waitForProcess with Process.terminationHandler bridge #1)

Proposal

Migrate to swift-subprocess, the swiftlang-org package designed exactly for this use case. Suggested by @just-doit in #4 along with the relevant Apple Developer Forums thread: https://developer.apple.com/forums/thread/690310.

What goes away

• makeProcess, attachDevNullStdin, waitForProcess, startDraining, awaitDrain (the entire helper layer)
• The continuation + polling loop
• Manual /dev/null stdin management
• Manual pipe drain and close
• GitError.processLaunchFailed (subsumed by Subprocess's error type)

What stays

• gitPath resolution, defaultTimeout, networkTimeout
• validateRepository, repositoryName, currentBranch, and every higher-level command method that wraps run
• The actor boundary on GitService

Acceptance criteria

• swift-subprocess added as the project's first SPM dependency (Xcode → File → Add Package Dependencies)
• GitService.run reduced to ~10 lines using Subprocess.run(...)
• All existing call sites work unchanged (signature preserved)
• Build clean, SwiftLint clean
• Manual smoke test: open a repo with > 64KB of git log output (the case from Loading a repository may timeout #4) and confirm no timeout

Notes

• swift-subprocess is pre-1.0 as of writing; pin to a specific version in the Xcode dependency dialog so future API changes don't surprise us
• This obsoletes Replace polling waitForProcess with Process.terminationHandler bridge #1 (the polling loop helper goes away entirely)