fix: fsync state file before rename to survive crashes

[ming-14]

Problem

If the opencode process terminates unexpectedly (crash, power loss, system restart), every subsequent prompt fails with:

Failed to send prompt / Unexpected server error

Server log shows:

StateDecodeError: JSON Parse error: Unrecognized token '\u0000'

Root cause: the state file goals.json (%APPDATA%/opencode-goal-plugin/goals.json) was found zero-filled — correct file size but all bytes \u0000.

Why

writeStateEffect (src/state.ts) wrote to a temp file and immediately renamed it into place without an intervening fsync:

1. writeFile(tmp, JSON.stringify(state, null, 2) + "\n")
2. rename(tmp, file)

On crash, the rename metadata could be committed while the data blocks were still in the OS page cache and never flushed to disk, leaving a zero-filled file after restart. A leftover .tmp file with intact contents further confirms the write was interrupted.

Fix

fsync the temp file before renaming it into place:

const handle = await open(tmp, "w", 0o600)
try {
  await handle.writeFile(JSON.stringify(state, null, 2) + "\n")
  await handle.sync()   // flush to disk before rename
} finally {
  await handle.close()
}
await rename(tmp, file)

Validation

• bun run typecheck / bun run lint / bun run build pass
• bun test: 93 pass; the only failing test (writes state with owner-only file permissions) is a pre-existing Windows chmod semantics issue, failing before this change
• 3 timer-based server tests fail intermittently and pass on re-run; unrelated to this change

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AI Assistance: This pull request was created with the assistance of AI (opencode using deepseek-v4).