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

PII redaction: build_session_context_prompt() now checks the plugin
registry's pii_safe flag in addition to the hardcoded _PII_SAFE_PLATFORMS
frozenset. Plugin platforms that set pii_safe=True (e.g. phone-based
messaging bridges) get their user IDs redacted before LLM context.

Token empty warnings: the empty-token diagnostic at config load now
checks the plugin registry's required_env when a platform isn't in the
hardcoded _token_env_names dict. Catches 'enabled but empty' for
plugin platforms too.

# Conflicts:
#	cron/scheduler.py
#	tools/send_message_tool.py

…, 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

Plugin platforms now get full toolset support without any entries in
toolsets.py.

tools_config._get_platform_tools(): Falls back to 'hermes-<name>'
  when the platform isn't in the static PLATFORMS dict. No more
  KeyError for plugin platforms.

toolsets.resolve_toolset(): Auto-generates a toolset for plugin
  platforms (hermes-<name>) containing _HERMES_CORE_TOOLS plus any
  tools the plugin registered into a matching toolset name. This means
  a plugin can call ctx.register_tool(toolset='irc', ...) and those
  tools will be included in the hermes-irc toolset automatically.

webhook.py: Registry-aware cross-platform delivery.
run_agent.py: Platform hints from plugin registry.
IRC adapter: Token lock + platform hint.
Removed dead token-empty-warning extension.
Updated docs.