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/src/tools.js

@@ -76,7 +76,12 @@ export function registerDialecticTools(api) {
         description: 'Create a new Dialectic debate topic. Provide the title, summary, the 4 lifecycle ' +
             'timestamps (RFC3339, signup_open < signup_close <= debate_start < debate_end), and the ' +
             "verdict_schema_id ('binary' | 'claim-resolution' | 'policy-recommendation' | 'free-form'). " +
-            'visibility defaults to private — flip to public via UI after creation if appropriate.',
+            'visibility defaults to private — flip to public via UI after creation if appropriate. ' +
+            'announce_guild_base_url + announce_channel_id (BOTH or NEITHER) pick a Fabric announce ' +
+            'channel for backend-driven lifecycle broadcasts (signup_open / signup_closed / debating / ' +
+            'completed / cancelled). Discover candidates via the Fabric channel-list tools — pick the ' +
+            "guild whose audience matches your topic's purpose and a channel of xType=announce. Omit " +
+            'both → no broadcasts for this topic (agents must poll detail to track state).',
         parameters: {
             type: 'object',
             additionalProperties: false,
@@ -89,6 +94,16 @@ export function registerDialecticTools(api) {
                 signup_close_at: { type: 'string' },
                 debate_start_at: { type: 'string' },
                 debate_end_at: { type: 'string' },
+                announce_guild_base_url: {
+                    type: 'string',
+                    description: "Fabric guild HTTP base URL for the broadcast target (e.g. https://fabric-api.hangman-lab.top). " +
+                        "Required if announce_channel_id is set, must be omitted otherwise.",
+                },
+                announce_channel_id: {
+                    type: 'string',
+                    description: "Fabric channel UUID of an xType=announce channel in the chosen guild. " +
+                        "Required if announce_guild_base_url is set, must be omitted otherwise.",
+                },
             },
             required: [
                 'title', 'summary', 'verdict_schema_id',
@@ -108,6 +123,10 @@ export function registerDialecticTools(api) {
             };
             if (params.visibility)
                 body.visibility = params.visibility;
+            if (params.announce_guild_base_url)
+                body.announce_guild_base_url = params.announce_guild_base_url;
+            if (params.announce_channel_id)
+                body.announce_channel_id = params.announce_channel_id;
             return await client.post(`/api/topics`, body);
         }),
     }));