fix(db): fail closed when WAL valve cannot checkpoint past caps (#1539) (#1751)

sync() armed the WAL valve but never called backpressure(), so daemon
catch-up could grow the WAL without bound while query-pool readers pinned
frames. Wire the writer pause into sync store + batched resolution, and
abort with WalValveAbortError after parked backfills fail past the
documented hard/file caps instead of disabling parking for 60s.

Co-authored-by: Colby McHenry <colbymchenry@users.noreply.github.com>

CHANGELOG.md

@@ -137,6 +137,8 @@ and adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 #### MCP / indexing
 
+- **Live sync no longer lets the write-ahead log grow without a bound when a reader is holding it open (#1539).** Incremental sync now uses the same writer pause that full indexing already used, and if checkpointing still cannot finish once the log is past its documented size limit — typically because the query pool is reading at the same time — sync stops with a clear error instead of keeping writing until the disk fills. The previous behaviour could leave a multi-tens-of-gigabyte log beside a few-gigabyte index on a large project. Close concurrent readers and retry, or raise `CODEGRAPH_WAL_VALVE_MB` if the limit is too tight for the project.
+
 - **A second `codegraph serve --mcp` on the same project no longer silently kills auto-sync (#1740).** Direct mode (`CODEGRAPH_NO_DAEMON=1` or proxy→in-process fallback) now takes an exclusive `.codegraph/writer.pid` lock; a second writer exits immediately with guidance to stop the other server or unset `CODEGRAPH_NO_DAEMON` so clients share the daemon. The shared daemon already multiplexes N clients onto one watcher — this closes the same-OS dual-direct gap the docs warned about for Windows/WSL but did not guard.
 
 - Indexing no longer checks whether files outside your project exist. A relative import that points above the project directory (`../../something`) made CodeGraph probe that location on disk while resolving it. Nothing outside the project was ever read, and no such file was ever added to the index or linked to, but the check itself should not have happened — such an import now simply resolves to nothing. Symlinks inside your project that point at code kept elsewhere are unaffected and still index as before. Thanks @ErQrYfkrju. (#1631)