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.

apps/cli/src/entrypoints/cli.ts

@@ -120,7 +120,7 @@ Topic  (conversation scope, v0.2.0)
   claudemesh topic history <t>     fetch message history [--limit --before]
   claudemesh topic read <topic>    mark all as read
   claudemesh topic tail <topic>    live SSE tail [--limit --forward-only]
-  claudemesh topic post <t> <msg>  encrypted REST post (v0.3.0 v2)
+  claudemesh topic post <t> <msg>  encrypted REST post (v0.3.0 v2) [--reply-to <id>]
   claudemesh send "#topic" "msg"   send to a topic (WS path, v1 plaintext)
   claudemesh member list           mesh roster with online state [--online]
   claudemesh notification list     recent @-mentions of you [--since <ISO>]
@@ -592,6 +592,7 @@ async function main(): Promise<void> {
           mesh: flags.mesh as string,
           json: !!flags.json,
           plaintext: !!flags.plaintext,
+          replyTo: (flags["reply-to"] as string) || (flags.replyTo as string),
         };
         const message = positionals.slice(2).join(" ");
         const { runTopicPost } = await import("~/commands/topic-post.js");