Phase 2 sweep: migrate src/snapshot/index.ts from raw ChildProcessSpawner to AppProcess.

Stacked on #27178.

Migrated call sites

All non-stdin git invocations now go through appProcess.run(ChildProcess.make("git", ...)) via a thin git helper that translates RunResult -> the existing GitResult shape. Uses interpretExitCode: () => "success" so non-zero exits flow through (preserving the prior behavior where the helper swallowed errors via Effect.catch and returned { code: 1, ... }).

Call sites flowing through the new git helper:

• excludes() — git rev-parse --git-path info/exclude
• add() — git diff-files --name-only -z, git ls-files --others --exclude-standard -z
• cleanup() — git gc --prune=<duration>
• track() — git init, git config core.* x4, git write-tree
• patch() — git diff --cached --no-ext-diff --name-only <hash>
• restore() — git read-tree <snapshot>, git checkout-index -a -f
• revert() — git checkout, git ls-tree (single + batched paths)
• diff() — git diff --cached --no-ext-diff <hash>
• diffFull() — git show <ref>:<file> (per-file fallback), git diff --name-status, git diff --numstat

Stayed on raw spawn (appProcess.spawn)

Four sites feed bytes into git's stdin via Stream, and AppProcess.run does not yet expose a stdin-Stream API. They now use appProcess.spawn(...) (inherited platform method) — same shape as before, no behavior change:

• ignore() — git check-ignore --stdin -z (NUL-delimited path list)
• drop() — git rm --cached --pathspec-from-file=- --pathspec-file-nul
• stage() — git add --all --sparse --pathspec-from-file=- --pathspec-file-nul
• diffFull() load() — git cat-file --batch (newline-delimited ref list)

These are isolated in a gitWithStdin helper alongside git to make the distinction explicit at the call site.

AppProcess API feedback

A runStdin(command, { stdin: Stream<Uint8Array> }) helper (or a stdin field on RunOptions) would let these last four sites migrate. Filing as future work — out of scope for this PR.

LOC delta

+45 / -14 in a single file (src/snapshot/index.ts).

Verification

• bun run test test/snapshot/ -> 52 pass, 1 skip, 0 fail
• bun run test test/session/snapshot-tool-race.test.ts -> 1 pass, 0 fail
• bun run typecheck -> 0 errors