snapshot: corrupt index causes repeated capture failures and unresponsive service

Summary

Snapshot capture writes into per-workspace snapshot git repositories under
~/.local/share/opencode/snapshot/<project.id>/<hash>. When the service is
restarted (including automatic next channel updates) mid-capture, the
repository's .git/index is left corrupt (signature 0x00000000). From then
on every capture attempt fails with Git.OperationError: hatalı imza 0x00000000 / fatal: indeks dosyası hasarlı, producing thousands of warnings
per hour and eventually making the web UI (and service) unresponsive after
roughly 10-30 conversation steps.

Environment

• opencode version: 0.0.0-next-17400 (observed failures also on 0.0.0-next-17292..17296)
• OS: Linux 7.0.0-28-generic #28~24.04.1-Ubuntu SMP x86_64
• Terminal: TERM=xterm-256color
• Shell: /bin/bash
• Install/channel: npm, next (auto-update observed: 13:57Z, 14:07Z, 14:17Z, 14:35Z, 14:46Z)
• Active plugins: /home/ugur/Projects/opencode-omni/dist/plugin.js, /home/ugur/Projects/opencode-rust-coder

Reproduction

1. Trigger a service restart while a session is active (e.g. opencode2 service restart, or wait for an automatic next update which restarts the service).
2. Send a message in the web UI / any session. The snapshot capture runs per event and writes the snapshot repo index.
3. If the restart interrupted a capture, subsequent captures keep failing: each event logs failed to capture snapshot with Git.OperationError: error: hatalı imza 0x00000000\nfatal: indeks dosyası hasarlı.
4. After roughly 10-30 steps (5-20 minutes), the web UI stops responding.

Expected Behavior

• Snapshot index writes should be atomic (temp file + rename) and/or capture should self-heal a corrupt index (e.g. recreate it) instead of failing forever.
• A corrupt index should not accumulate failures to the point where the service becomes unresponsive.

Actual Behavior

• packages/core/src/snapshot.ts has no atomic write pattern (no writeFile/rename) and no self-heal for a corrupt index.
• Capture runs git subprocesses (packages/core/src/git.ts:192 run(cwd, proc)), so an interrupted write leaves a partial index.
• 13,780 failed to capture snapshot warnings accumulated between 14:25Z and 19:24Z on 2026-08-12; the web UI became unresponsive in this window.
• Manual repair (rm .git/index && git reset in the affected snapshot repos) stopped the failures completely: 0 failures after 19:24:33Z over 8+ hours.

Additional Context

• Snapshot repos live at ~/.local/share/opencode/snapshot/<project.id>/<hash>; the git repository root is the session dir itself (branches/config/HEAD/index present, no nested .git).
• Snapshot can be disabled via config ("snapshots": false, checked via Config.latest(config, "snapshots") !== false in snapshot.ts), but default-on should be resilient.
• The affected repo index signature was 0x00000000 (4-byte check of the index file).
• Workaround that resolved it: recreate the corrupt index files in the snapshot directories (rm index && git reset) and restart the service.