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/platform_registry.py

@@ -67,6 +67,28 @@ class PlatformEntry:
     # "builtin" or "plugin"
     source: str = "plugin"
 
+    # ── Auth env var names (for _is_user_authorized integration) ──
+    # E.g. "IRC_ALLOWED_USERS" — checked for comma-separated user IDs.
+    allowed_users_env: str = ""
+    # E.g. "IRC_ALLOW_ALL_USERS" — if truthy, all users authorized.
+    allow_all_env: str = ""
+
+    # ── Message limits ──
+    # Max message length for smart-chunking.  0 = no limit.
+    max_message_length: int = 0
+
+    # ── Privacy ──
+    # If True, session descriptions redact PII (phone numbers, etc.)
+    pii_safe: bool = False
+
+    # ── Display ──
+    # Emoji for CLI/gateway display (e.g. "💬")
+    emoji: str = "🔌"
+
+    # Whether this platform should appear in _UPDATE_ALLOWED_PLATFORMS
+    # (allows /update command from this platform).
+    allow_update_command: bool = True
+
 
 class PlatformRegistry:
     """Central registry of platform adapters.