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

plugins/platforms/irc/adapter.py

@@ -490,4 +490,14 @@ def register(ctx):
         validate_config=validate_config,
         required_env=["IRC_SERVER", "IRC_CHANNEL", "IRC_NICKNAME"],
         install_hint="No extra packages needed (stdlib only)",
+        # Auth env vars for _is_user_authorized() integration
+        allowed_users_env="IRC_ALLOWED_USERS",
+        allow_all_env="IRC_ALLOW_ALL_USERS",
+        # IRC line limit after protocol overhead
+        max_message_length=450,
+        # Display
+        emoji="💬",
+        # IRC doesn't have phone numbers to redact
+        pii_safe=False,
+        allow_update_command=True,
     )