Summary

Adds apply_yaml_config_fn to PlatformEntry so platform plugins can own their YAML→env config bridges instead of forcing core gateway/config.py to know every platform's schema. Pure addition — no behavior change for existing platforms.

Closes #24836. Refs #3823, #24356.

Why

gateway/config.py currently has hardcoded YAML→env bridges for 8 platforms (discord, telegram, whatsapp, slack, dingtalk, mattermost, matrix, feishu, ~252 LOC). When a platform migrates to the bundled plugin system (e.g. Discord in #24356), its bridge stays stuck in core. Third-party plugin platforms can't bridge YAML keys to env at all without monkey-patching load_gateway_config. The existing env_enablement_fn hook (#21306, #21331) handles env→extras seeding but not YAML→env, which is the symmetric case this hook covers.

What changed

hermes_cli/plugins.py::register_platform() already forwards **entry_kwargs to PlatformEntry, so plugins can pass apply_yaml_config_fn= immediately with no plugin-side core change.

Order of operations

load_gateway_config():
  1. Generic shared-key loop (unchanged)         — handles unauthorized_dm_behavior,
                                                    notice_delivery, reply_prefix,
                                                    require_mention, etc. across all
                                                    Platform enum members.
  2. apply_yaml_config_fn dispatch (NEW)         — platform-specific YAML keys.
  3. Legacy per-platform hardcoded blocks        — still run; their `not os.getenv(...)`
                                                    guards make them no-ops for any env
                                                    var the hook already set, so env
                                                    > YAML precedence is preserved.
  4. GatewayConfig.from_dict + _apply_env_overrides

This order means each of the 8 hardcoded platforms can migrate independently in follow-up PRs without breaking anything, and migration is net-negative LOC per platform.

Example usage (for a future plugin migration PR)

import os

def _apply_yaml_config(yaml_cfg: dict, platform_cfg: dict) -> dict | None:
    if "require_mention" in platform_cfg and not os.getenv("MY_PLATFORM_REQUIRE_MENTION"):
        os.environ["MY_PLATFORM_REQUIRE_MENTION"] = str(platform_cfg["require_mention"]).lower()
    allowed = platform_cfg.get("allowed_channels")
    if allowed is not None and not os.getenv("MY_PLATFORM_ALLOWED_CHANNELS"):
        if isinstance(allowed, list):
            allowed = ",".join(str(v) for v in allowed)
        os.environ["MY_PLATFORM_ALLOWED_CHANNELS"] = str(allowed)
    return None  # nothing extra to seed into PlatformConfig.extra

def register(ctx):
    ctx.register_platform(
        name="my_platform",
        ...,
        apply_yaml_config_fn=_apply_yaml_config,
    )

Test plan

• bash scripts/run_tests.sh tests/gateway/test_platform_registry.py → 42 passed (32 existing + 10 new).
• bash scripts/run_tests.sh tests/gateway/test_discord_reply_mode.py tests/gateway/test_slack_mention.py tests/gateway/test_whatsapp_reply_prefix.py tests/gateway/test_whatsapp_group_gating.py → 118 passed (every YAML-loading test in the gateway suite).
• Full tests/gateway/ → 5367 passed, 9 failures all pre-existing on origin/main (Discord document-handling + Matrix E2EE — verified by stashing this PR and re-running on stock main).
• E2E with real load_gateway_config(): registered a temporary plugin platform with a YAML→env hook in a tmp HERMES_HOME, wrote a config.yaml exercising both the new hook AND the existing Discord hardcoded block, and verified:
  • Discord legacy block still bridges discord.require_mention and discord.allowed_channels to env vars.
  • Plugin hook successfully sets its own env vars and seeds PlatformConfig.extra.
  • Both coexist cleanly.

Out of scope (future PRs)

• Migrating any specific platform's hardcoded bridge to use the hook — each is independent and net-negative LOC. Discord first, riding on refactor(gateway): migrate Discord adapter to bundled plugin #24356.
• The other Discord-shaped concerns called out in refactor(gateway): migrate Discord adapter to bundled plugin #24356 (Platform.DISCORD enum, allowlist maps, _UPDATE_ALLOWED_PLATFORMS, voice-mode adapter access) — independent generic-registry concerns deserving their own scoping issues.