docs: say what the WAL fix bounds — the log's resting size, never the index (#1431)

The 1.6.0 notes said the write-ahead log is "capped", which reads as a
limit on how much can be indexed. It bounds only the log's resting size
(64 MB default, CODEGRAPH_WAL_HEAL_MB) and folds a killed session's
leftover back into the index; a large repository's log still grows in
proportion to its index while it is built. Say so in both entries, and
document the two knobs in the README's troubleshooting section.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MC52FSFLtKtDCLqT81tZYG

README.md

@@ -876,6 +876,8 @@ Framework routing is validated the same way, on a canonical app per framework: E
 
 **Sharing one checkout between Windows and WSL** — Don't point both at the same `.codegraph/`: the background-server lock and the SQLite index are tied to the OS that wrote them, and SQLite locking across the WSL2/Windows filesystem boundary is unreliable. Give each side its own index in the same tree by setting `CODEGRAPH_DIR` to a distinct name on one of them — e.g. `CODEGRAPH_DIR=.codegraph-win` on Windows, leaving WSL on the default `.codegraph`. CodeGraph skips any sibling `.codegraph-*` directory when indexing and watching, so the two never trip over each other.
 
+**Very large repositories (hundreds of thousands of files), or a large `.codegraph/codegraph.db-wal` file** — The `-wal` file is SQLite's write-ahead log: writes waiting to be folded into `codegraph.db`. While a big index is being built, CodeGraph lets it grow in proportion to the index (soft threshold = the larger of 256 MB and a quarter of the index size, up to 2 GB) before folding it back, because folding too often is what made large indexes slow on ordinary disks. At rest it is trimmed to 64 MB, and a leftover from a killed session is folded and trimmed the next time the project opens — the index itself has no size limit. Two environment variables tune this: `CODEGRAPH_WAL_VALVE_MB` (the soft threshold during indexing) and `CODEGRAPH_WAL_HEAL_MB` (the resting size and the trim threshold). `CODEGRAPH_WAL_VALVE_DEBUG=1` prints every decision to stderr.
+
 ## License
 
 MIT