* fix(config): revalidate the extensions config singleton by file signature
`get_extensions_config()` parsed `extensions_config.json` once per process
and only the process that handled a write ever reloaded it. With several
uvicorn workers, or several Pods sharing one volume, every other process
kept its startup copy: the MCP servers the agent could use and the
local-bash absolute path allowlist derived from the filesystem MCP server
differed between replicas until each one restarted.
The singleton now records the resolved path and the file's
(mtime, size, sha256) signature and reloads when either changes, like
`get_app_config()` does for `config.yaml` and `mcp/cache.py` does for the
tools cache. After a successful load, a revision that cannot be loaded
(file gone, truncated, invalid) keeps the last-known-good configuration
and is logged once with the exception type only; the first load still
raises. `set_extensions_config()` pins the injected instance until a
reload or reset. The cached AppConfig's `extensions` snapshot is built
from the singleton instance and refreshes when that instance changes, so
it inherits the same fail-soft policy instead of parsing the file again.
* docs(changelog): link the extensions config freshness entry to #6386
* fix(config): follow an explicitly reloaded extensions config path
`reload_extensions_config(config_path=X)` recorded X, but the freshness
probe re-resolved the default path, so the next read either flipped the
cache back to the default file or pinned a stale copy of X. The cache now
remembers the explicit path and both the probe and later reloads follow
it until a reset or an argument-less reload. Also shortens the pointer in
backend/AGENTS.md so the nested guidance chains stay under the hard
size limit enforced by the agent-guidance check.
* docs: keep backend guidance within the inherited size budget
* fix(config): reload the probed extensions config path
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Co-authored-by: Willem Jiang <willem.jiang@gmail.com>
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