feat: final platform plugin parity — webhook delivery, platform hints, docs

Closes remaining functional gaps and adds documentation.

## Functional fixes

webhook.py: Cross-platform delivery now checks the plugin registry
  for unknown platform names instead of hardcoding 15 names in a tuple.
  Plugin platforms can receive webhook-routed deliveries.

prompt_builder: Platform hints (system prompt LLM guidance) now fall
  back to the plugin registry's platform_hint field. Plugin platforms
  can tell the LLM 'you're on IRC, no markdown.'

PlatformEntry: Added platform_hint field for LLM guidance injection.

IRC adapter: Added acquire_scoped_lock/release_scoped_lock in
  connect/disconnect to prevent two profiles from using the same IRC
  identity. Added platform_hint for IRC-specific LLM guidance.

Removed dead token-empty-warning extension for plugin platforms
  (plugin adapters handle their own env vars via check_fn).

## Documentation

website/docs/developer-guide/adding-platform-adapters.md:
  - Added 'Plugin Path (Recommended)' section with full code examples,
    PLUGIN.yaml template, config.yaml examples, and a table showing all
    18 integration points the plugin system handles automatically
  - Renamed built-in checklist to clarify it's for core contributors

gateway/platforms/ADDING_A_PLATFORM.md:
  - Added Plugin Path section pointing to the reference implementation
    and full docs guide
  - Clarified built-in path is for core contributors only

gateway/platform_registry.py

@@ -89,6 +89,11 @@ class PlatformEntry:
     # (allows /update command from this platform).
     allow_update_command: bool = True
 
+    # ── LLM guidance ──
+    # Platform hint injected into the system prompt (e.g. "You are on IRC.
+    # Do not use markdown.").  Empty string = no hint.
+    platform_hint: str = ""
+
 
 class PlatformRegistry:
     """Central registry of platform adapters.