feat(photon): group-chat mention gating for full channel parity

Adds the last missing parity piece vs the established channels: group
chats can be made opt-in via a mention wake word, exactly like the
BlueBubbles iMessage channel.

- require_mention + mention_patterns, read from config.extra (config.yaml
  via the generic gateway bridge) or PHOTON_REQUIRE_MENTION /
  PHOTON_MENTION_PATTERNS env vars. Same shapes BlueBubbles accepts
  (list / JSON / comma / newline), same default Hermes wake words.
- _dispatch_inbound drops unmatched group messages and strips the leading
  wake word from matched ones; DMs are never gated.
- plugin.yaml + docs document both knobs and the config.yaml form.
- New test_mention_gating.py (8 tests): default-off, group drop/pass,
  wake-word strip, DM bypass, custom patterns, env comma-list, invalid
  regex skip.

The config.yaml -> extra bridge needed no core change — the generic
shared-key loop in gateway/config.py already iterates plugin platforms
(_shared_loop_targets += plugin_entries()), so require_mention /
mention_patterns flow through automatically.

Note: outbound media is the one capability Photon still can't reach —
Photon exposes no HTTP send-attachment endpoint yet (documented API
limitation), so the sidecar can't send files. Not faked.

Validation: 34/34 photon tests; E2E confirms config.yaml require_mention
+ custom mention_patterns bridge through load_gateway_config into a live
adapter and gate/strip correctly.

plugins/platforms/photon/plugin.yaml

@@ -73,6 +73,14 @@ optional_env:
     description: "Allow any sender to trigger the bot (dev only — disables allowlist)"
     prompt: "Allow all users? (true/false)"
     password: false
+  - name: PHOTON_REQUIRE_MENTION
+    description: "Ignore group-chat messages unless they match a mention wake word (true/false, default false)"
+    prompt: "Require a mention in group chats?"
+    password: false
+  - name: PHOTON_MENTION_PATTERNS
+    description: "Mention wake-word regexes for group chats (JSON list or comma/newline-separated; defaults to Hermes wake words)"
+    prompt: "Group mention patterns"
+    password: false
   - name: PHOTON_HOME_CHANNEL
     description: "Default Spectrum space ID for cron / notification delivery"
     prompt: "Home space ID"