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* fix(mcp): invalidate tools cache on config content + path, not just newer mtime The MCP tools cache invalidated only on a strict extensions-config mtime `>` comparison and tracked no resolved config path, so `_is_cache_stale()` missed: - content changes with an unchanged mtime (same-second edits; object-store / network mounts that do not bump mtime); - content changes with a backward mtime (git checkout, cp -p / backup restore, tar / rsync preserving timestamps); - a resolved-path switch to a different config file with mtime <= the recorded value (structurally invisible — no path was tracked at all). On multi-worker (uvicorn/gunicorn) or stale-mtime deployments this leaves the LangGraph-embedded runtime and every non-writer worker serving stale MCP tools after `PUT /api/mcp/config`, breaking the module's documented promise that changes made through the Gateway API are reflected in the embedded runtime. Record the resolved config path and a `(mtime, size, sha256)` content signature at initialization and invalidate when the path OR the signature differs (`!=`), mirroring `config/app_config.py::get_app_config()` so the two runtime-editable config files share one content-based staleness signal. The per-call stat was already paid, and the small-file sha256 matches the cost app_config pays per request. The "config missing / not yet initialized" no-op behavior and the cache reset endpoint are preserved. Adds backend/tests/test_mcp_cache.py covering all three failure modes plus unchanged-file and forward-edit sanity cases. Also folds in an incidental backend/AGENTS.md doc-sync: the restart-required field list was missing `scheduler` and `run_ownership`, both present in reload_boundary.py::STARTUP_ONLY_FIELDS. * test(mcp): pin cache staleness contracts raised in review Two review observations on the content-signature cache fix, both raised as non-blocking design questions rather than bugs: - Whether relying on mtime+size alone (skipping the sha256) could ever be "optimized" back in, reopening the narrow same-second / identical-length swap gap the signature was built to close. - Whether the extensions config being deleted entirely after a successful init leaves the cache in a defined state, since current_signature flips to None and _is_cache_stale() returns False. Neither is a behavior change: both were already the intended contract, preserved verbatim from the pre-fix mtime-only code (which also returned False once the file could no longer be stat-ed). Record the reasoning inline and add regression tests that pin each contract so a future change cannot alter either silently: - test_same_mtime_same_size_swap_is_stale: a same-length server-name swap that leaves mtime AND size unchanged (the precise scenario from review, sharper than the existing same-mtime test, which also changes size) is still caught only because the sha256 is computed unconditionally. - test_config_deleted_after_init_is_not_stale: deleting the config file after init keeps the cache serving its last-known-good MCP tools instead of invalidating into an unconfigured state. Both new tests were confirmed to fail against a deliberately reintroduced version of the regression they guard (hash short-circuit / removed None-guard), then confirmed to pass against the real code.