fix(config): make the reload boundary discoverable from code (#3144) (#3153)

* fix(config): make the reload boundary discoverable from code, not just docs

Closes #3144.

The hot-reload contract — per-run fields are resolved through
`get_app_config()` on every request, infrastructure fields snapshot at
gateway startup — landed in `backend/CLAUDE.md` as part of #3131. A
maintainer reading `get_config()` or an `AppConfig` field still had to
context-switch to that document to know which fields require a process
restart, and there was no enforcement that the prose list stayed in
sync with the code.

This commit moves the boundary to a machine-readable single source of
truth and surfaces it where the code lives:

- New `deerflow.config.reload_boundary` module owns the registry of
  restart-required fields (`STARTUP_ONLY_FIELDS`) and a tiny helper
  API (`is_startup_only_field`, `iter_startup_only_field_paths`,
  `format_field_description`). The standardised `"startup-only:"`
  prefix is exported as `STARTUP_ONLY_PREFIX` so future scanners /
  lint hooks / doc generators can pivot off it without re-parsing
  prose.
- `AppConfig`'s `database`, `checkpointer`, `run_events`,
  `stream_bridge`, `sandbox`, and `log_level` fields now build their
  `Field(description=...)` from `format_field_description(...)`. The
  same text shows up in IDE hover (Pydantic v2 exposes `description`
  via `model_fields[...]`).
- `channels` is restart-required too but lives outside the AppConfig
  Pydantic schema (the config section is consumed directly by
  `start_channel_service`). The registry owns it so the boundary is
  not split between two places.
- `get_config()` docstring points to the registry instead of leaving
  the reader to find `CLAUDE.md`. The `CLAUDE.md` table collapses to
  a one-liner pointing back at `reload_boundary.py` so the boundary
  has one canonical location, not two.

Drift coverage in `tests/test_reload_boundary.py`:

- Every registered field has a non-trivial reason.
- Iterator / membership helpers stay in sync with the dict.
- Every registry entry that maps to an `AppConfig` field also carries
  the `"startup-only:"` prefix in the schema (catches "forgot to
  update the schema").
- Reverse drift: any AppConfig field whose description starts with
  the prefix must be registered (catches "marked restart-required in
  the schema but forgot the registry").
- The runtime introspection that IDE hover depends on
  (`AppConfig.model_fields["database"].description`) is pinned, so a
  future Pydantic upgrade or schema swap that breaks the hover surface
  shows up as a test failure rather than a silent regression.

Refs: bytedance/deer-flow#3138 (split summary), #3107 (origin), #3131
(prior boundary fix in prose form).

* fix(config): preserve field doc and correct log_level reload reason

Two follow-ups on the PR #3153 review:

1. The `log_level` STARTUP_ONLY_FIELDS reason previously claimed
   `apply_logging_level()` mutates the root logger level. It does not:
   only the `deerflow` / `app` logger levels are set, and root handler
   thresholds are conditionally lowered so messages from those loggers
   can propagate. Reword to match the actual behavior so operators
   reading IDE hover get accurate restart guidance.

2. `format_field_description(field_path)` was the sole `Field(description=)`
   for every restart-required field, which silently overwrote the
   original human-facing documentation — most visibly the `log_level`
   field that used to list debug/info/warning/error and clarify that
   third-party libraries are not affected. Extend the helper with a
   keyword-only `field_doc` parameter that composes the startup-only
   marker with the original prose so IDE hover documents both *why*
   the field is restart-required and *what* it actually accepts.
   Updated all six restart-required AppConfig fields (`log_level`,
   `database`, `sandbox`, `run_events`, `checkpointer`, `stream_bridge`)
   to pass their original descriptions through the helper.

Tests: two new cases in `test_reload_boundary.py` pin (a) the helper
composition and (b) every AppConfig restart-required field still
surfaces a recognisable substring of its original documentation.

---------

Co-authored-by: Willem Jiang <willem.jiang@gmail.com>

backend/app/gateway/deps.py

@@ -119,6 +119,16 @@ def get_config() -> AppConfig:
     split-brain where the worker / lead-agent thread saw a stale startup
     snapshot.
 
+    Hot-reload boundary: fields backed by startup-time singletons
+    (engines, sandbox provider, IM channels, logging handler) require a
+    process restart to change at runtime. The authoritative list lives in
+    :mod:`deerflow.config.reload_boundary` and is mirrored by the
+    standardised ``"startup-only:"`` prefix on the matching
+    ``Field(description=...)`` in :class:`AppConfig` — IDE hover on those
+    fields will surface the boundary inline. See
+    ``backend/CLAUDE.md`` "Config Hot-Reload Boundary" for the operator
+    summary.
+
     Any failure to materialise the config (missing file, permission denied,
     YAML parse error, validation error) is reported as 503 — semantically
     "the gateway cannot serve requests without a usable configuration" — and