fix(opencode): respect repository git semantics - #41962

Summary

• stop overriding core.autocrlf and core.symlinks for commands against user repositories
• make private snapshot repositories mirror the source repository's effective line-ending and symlink semantics
• reconcile existing snapshot stores when those semantics differ, while preserving historical snapshot objects
• seed snapshots from the active worktree index and fail safely instead of deleting files when snapshot lookup fails

Design

Normal repository commands inherit Git's effective repository configuration. Snapshot repositories remain OpenCode-owned and keep operational settings such as long-path and fsmonitor handling, but their working-tree semantics are resolved from the source repository and stored behind a versioned fingerprint. A changed or legacy fingerprint reseeds the private index from the active worktree before capture.

Checks

• bun test test/git/git.test.ts test/snapshot/snapshot.test.ts --timeout 30000 (69 pass, 1 existing skip)
• bun x oxlint ... (0 errors; existing warnings only)
• git diff --check
• bun typecheck attempted twice but did not complete within 10 minutes on the shared host

Requested by: @Hona (Hona via Slack)