refactor(plugins): add apply_yaml_config_fn registry hook

Lets platform plugins own their YAML→env config bridge instead of forcing
core gateway/config.py to know every platform's schema.

The hook receives the full parsed config.yaml and the platform's own
sub-dict, may mutate os.environ (env > YAML precedence preserved via the
standard `not os.getenv(...)` guards), and may return a dict to merge
into PlatformConfig.extra. It runs during load_gateway_config() after
the existing generic shared-key loop and before _apply_env_overrides(),
mirroring the env_enablement_fn dispatch pattern (#21306, #21331).

Pure addition — no behavior change for existing platforms. Each of the
eight platforms with hardcoded YAML→env blocks today (discord, telegram,
whatsapp, slack, dingtalk, mattermost, matrix, feishu, ~252 LOC in
gateway/config.py) can migrate in independent follow-up PRs; the
hardcoded blocks remain functional in the meantime, and their
`not os.getenv(...)` guards make them no-ops for any env var the hook
already set.

Test coverage: 10 new tests in tests/gateway/test_platform_registry.py
covering field default, callable acceptance, env mutation, extras
merge, both signature args, exception swallowing, missing/non-dict
sections, and env > YAML precedence.

Refs #3823, #24356.
Closes #24836.

gateway/platforms/ADDING_A_PLATFORM.md

@@ -21,6 +21,14 @@ status display, gateway setup, and more.
   constructed.  Without this, env-only setups don't surface in
   `hermes gateway status` or `get_connected_platforms()` until the SDK
   instantiates.
+- `apply_yaml_config_fn: (yaml_cfg, platform_cfg) -> Optional[dict]` —
+  translate this platform's `config.yaml` keys into env vars and/or seed
+  `PlatformConfig.extra` directly.  Lets a plugin own its YAML schema
+  instead of growing core `gateway/config.py` boilerplate per platform.
+  Mutating `os.environ` is allowed (use `not os.getenv(...)` guards to
+  preserve env > YAML precedence); the returned dict is merged into
+  `PlatformConfig.extra`.  Called during `load_gateway_config()` after
+  the generic shared-key loop and before `_apply_env_overrides()`.
 - `cron_deliver_env_var: str` — name of the `*_HOME_CHANNEL` env var.  When
   set, `deliver=<name>` cron jobs route to this var without editing
   `cron/scheduler.py`'s hardcoded sets.