refactor(snapshot): effectify SnapshotService

[kitlangton]

Summary

• Converts Snapshot from a promise-based namespace with Instance ALS reads to an Effect service (SnapshotService) on the Instances LayerMap
• Uses ChildProcessSpawner for git subprocess execution and Effect FileSystem for file operations (replaces Process.run and raw fs calls)
• Replaces Scheduler.register with Effect.forkScoped + Schedule.spaced for hourly cleanup
• Adds worktree to InstanceContext.Shape and Instance.current getter

Details

• Nothrow git helper that always returns { code, text, stderr }, preserving spawn failure details
• Parallelized before/after git show in diffFull via Effect.all({ concurrency: 2 })
• Promise facade preserved — zero changes needed at call sites
• Platform layers (NodeChildProcessSpawner, NodeFileSystem, NodePath) provided directly on the service layer
• Extracted repeated git config flags into GIT_CORE / GIT_CFG / GIT_CFG_QUOTE constants

Test plan

• All 43 existing snapshot tests pass (track, patch, revert, restore, diff, diffFull, worktrees, unicode, binary, symlinks, edge cases)
• Typecheck clean
• Verify extendEnv: true doesn't break git PATH resolution (behavioral difference from old Process.run env handling)
• Verify exists + mkdir in track() doesn't cause issues vs old atomic fs.mkdir return value check