feat(broker+api+cli): topic message reply-to threading (v0.3.1)

Adds a reply_to_id column (self-FK on topic_message) plus end-to-end
plumbing so a message can mark itself as a reply to a previous one in
the same topic.

- Schema: 0027_topic_message_reply_to.sql adds reply_to_id with
  ON DELETE SET NULL + index for backlink lookup.
- Broker: appendTopicMessage validates parent shares the topic, writes
  reply_to_id; topicHistory + topic_history_response surface it; WS
  push envelope now carries senderMemberId, senderName, topic name,
  reply_to_id, and message_id so recipients have everything they need
  to reply without a follow-up query.
- REST: POST /v1/messages accepts replyToId (validated server-side);
  GET /messages and SSE /stream emit it per row.
- CLI: \`topic post --reply-to <id|prefix>\` resolves prefixes against
  recent history; \`topic tail\` renders an "↳ in reply to <name>:
  <snippet>" line above replies and shows a copyable #shortid tag on
  every row.
- MCP push pipe: channel attributes now include from_pubkey,
  from_member_id, message_id, topic, reply_to_id — the recipient can
  thread a reply directly from the inbound notification.
- Skill + identity prompt updated to teach Claude how to use the new
  attributes for replies.

Bumped CLI to 1.9.0.

packages/db/src/schema/mesh.ts

@@ -1491,11 +1491,13 @@ export const meshTopicMessage = meshSchema.table(
      * a v2 message still resolves @-mentions correctly.
      */
     bodyVersion: integer().notNull().default(1),
+    replyToId: text("reply_to_id"),
     createdAt: timestamp().defaultNow().notNull(),
   },
   (t) => [
     index("topic_message_by_topic_time").on(t.topicId, t.createdAt),
     index("topic_message_by_version").on(t.bodyVersion),
+    index("topic_message_by_reply_to").on(t.replyToId),
   ],
 );