feat: complete plugin platform parity — all 12 integration points

Extends the platform plugin interface from Phase 1 to cover every
touchpoint where built-in platforms have hardcoded behavior.

## PlatformEntry extended fields
- allowed_users_env / allow_all_env: per-platform auth env vars
- max_message_length: smart-chunking for send_message tool
- pii_safe: session PII redaction flag
- emoji: CLI/gateway display
- allow_update_command: /update access control

## Functional fixes (Tier 1)

send_message tool (tools/send_message_tool.py):
- Replaced hardcoded platform_map dict with Platform() call
- Added _send_via_adapter() for plugin platforms — routes through
  live gateway adapter when available
- Registry-aware max message length for smart chunking

Cron delivery (cron/scheduler.py):
- Replaced hardcoded 15-entry platform_map with Platform() call
- Plugin platforms now work as cron delivery targets

User authorization (gateway/run.py _is_user_authorized):
- Registry fallback: checks PlatformEntry.allowed_users_env and
  allow_all_env when platform not in hardcoded maps
- Plugin platforms get per-platform auth support

## Integration fixes (Tier 2)

_UPDATE_ALLOWED_PLATFORMS: checks registry allow_update_command flag
Channel directory: includes plugin platforms in session enumeration
Orphaned config warning: descriptive message when plugin platform is
  in config but no plugin registered it
Gateway weakref: _gateway_runner_ref for cross-module adapter access

## UX completeness

hermes status: shows plugin platforms with (plugin) tag
hermes gateway setup: plugin platforms appear in menu with setup hints
hermes_cli/platforms.py: get_all_platforms() merges with registry,
  platform_label() falls back to registry for plugin names

## Tests
- 8 new tests (extended fields, cron resolution, platforms merge)
- Updated 3 tests for new Platform() based resolution
- 2829 passed, 24 pre-existing failures, zero new failures

gateway/channel_directory.py

@@ -86,6 +86,16 @@ def build_channel_directory(adapters: Dict[Any, Any]) -> Dict[str, Any]:
             continue
         platforms[plat_name] = _build_from_sessions(plat_name)
 
+    # Include plugin-registered platforms (dynamic enum members aren't in
+    # Platform.__members__, so the loop above misses them).
+    try:
+        from gateway.platform_registry import platform_registry
+        for entry in platform_registry.plugin_entries():
+            if entry.name not in _SKIP_SESSION_DISCOVERY and entry.name not in platforms:
+                platforms[entry.name] = _build_from_sessions(entry.name)
+    except Exception:
+        pass
+
     directory = {
         "updated_at": datetime.now().isoformat(),
         "platforms": platforms,