[code-yeongyu]

Fixes #4295

Summary

• forkFrom is atomic. The fork is built through replaceFileAtomically (temp file in the same directory, fsync, rename) instead of an exclusive header write followed by one appendFileSync per entry. A full disk part-way through now leaves no half-copied fork in the session list. The fork's name carries a fresh timestamp and session id, as before; no extra existence probe is added, because the RPC session path's no-sync audit admits no new blocking call. The two retired forkFrom ledger entries (writeFileSync, appendFileSync) are removed.
• A failed header catch-up keeps its prefix. _writeHeaderAsync writes the header and the first entries through an exclusive handle, then appends entries persisted while that handle closed. When that catch-up append failed, the cleanup deleted the whole new file, including the header and the entries already on disk. Now the file is kept and cut back to the bytes known whole, the transcript counts as flushed, and the entries the failed append carried are written, one line each, before the next append. A failure while the exclusive handle is still open removes the part-written file as before (fix(coding-agent): a refused delivery, header or release write never leaves a session stuck #2370).
• createBranchedSession already writes through the atomic _rewriteFile (fix(coding-agent): rewrite session files atomically so a failed migration keeps the original #2367). Its full-disk case is added as a guard test.

Evidence

• RED on main 96f8ac1: fork-full-disk.test.ts (three half-copied forks left behind) and the new session-manager-header-write.test.ts case (expected [] to deeply equal [ 'session', 'before' ]).
• GREEN at this head: test/session-manager (all files), session-manager-header-write and session-append-refused-mid-turn, 234/234. tsc --noEmit, oxlint, oxfmt --check, lint ratchet and the changelog gate pass.
• Simulated ENOSPC only (fs / fs/promises mocks with a byte budget); no real disk was filled.

Merge order (with bugwatch)

#4297 (A1) -> #4303 (B) -> #4300 (A2) -> #4305 (C) last. #4305 touches the same flushed-append block in SessionManager._persist as #4300, so it rebases after #4300 merges. #4305 carries Fixes #4295; the others are Part of #4295.