feat(tools): propose_topic gains announce_guild_base_url + announce_channel_id

Operator decision: backend env hard-coding a single guild/channel was
wrong. Center can host multiple guilds, each can have multiple
announce channels for different purposes. The proposing agent picks
which one this topic broadcasts to.

dialectic_propose_topic tool schema adds:
- announce_guild_base_url (string, optional)
- announce_channel_id    (string, optional)
- both or neither - one-of-two rejected at backend POST time
- omit both - topic has no broadcasts (agents must poll detail)

Tool description points agents at "Fabric channel-list tools" for
candidate discovery. Workflow docs spell out the discovery pattern.

plugin/index.js

@@ -3,7 +3,8 @@
  *
  * Tools: dialectic_list_topics, dialectic_topic_detail,
  *        dialectic_propose_topic, dialectic_signup,
- *        dialectic_post_argument, dialectic_view_verdict
+ *        dialectic_post_argument, dialectic_submit_verdict,
+ *        dialectic_view_verdict
  *
  * Loader gotchas (per [[reference-meridian-plugin-contract]]):
  *   - openclaw.plugin.json MUST declare `activation.onStartup: true`