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/commands/topic-post.ts

@@ -29,6 +29,8 @@ export interface TopicPostFlags {
   json?: boolean;
   /** Force v1 plaintext send even if the topic is encrypted. */
   plaintext?: boolean;
+  /** Reply-to message id (full or 8+ char prefix). */
+  replyTo?: string;
 }
 
 interface PostResponse {
@@ -37,6 +39,7 @@ interface PostResponse {
   topic: string;
   topicId: string;
   notifications: number;
+  replyToId?: string | null;
 }
 
 export async function runTopicPost(
@@ -101,6 +104,36 @@ export async function runTopicPost(
         }
       }
 
+      // Resolve reply-to: accept full id or 8+ char prefix by querying recent
+      // history once and matching. Server validates same-topic membership.
+      let replyToId: string | undefined;
+      if (flags.replyTo) {
+        if (flags.replyTo.length >= 16) {
+          replyToId = flags.replyTo;
+        } else if (flags.replyTo.length >= 6) {
+          const recent = await request<{
+            messages: Array<{ id: string }>;
+          }>({
+            path: `/api/v1/topics/${encodeURIComponent(cleanName)}/messages?limit=200`,
+            method: "GET",
+            token: secret,
+          });
+          const hit = recent.messages?.find((r) =>
+            r.id.startsWith(flags.replyTo!),
+          );
+          if (!hit) {
+            render.err(
+              `--reply-to ${flags.replyTo}: no recent message id starts with that prefix`,
+            );
+            return EXIT.INVALID_ARGS;
+          }
+          replyToId = hit.id;
+        } else {
+          render.err("--reply-to needs at least 6 characters of the message id");
+          return EXIT.INVALID_ARGS;
+        }
+      }
+
       const result = await request<PostResponse>({
         path: "/api/v1/messages",
         method: "POST",
@@ -111,6 +144,7 @@ export async function runTopicPost(
           nonce,
           bodyVersion,
           ...(mentions.length > 0 ? { mentions } : {}),
+          ...(replyToId ? { replyToId } : {}),
         },
       });
 
@@ -120,9 +154,12 @@ export async function runTopicPost(
       }
 
       const versionTag = bodyVersion === 2 ? green("🔒 v2") : dim("v1");
+      const replyTag = result.replyToId
+        ? `  ${dim("↳ " + result.replyToId.slice(0, 8))}`
+        : "";
       render.ok(
         "posted",
-        `${clay("#" + cleanName)}  ${versionTag}  ${dim(`(${result.notifications} mentions)`)}`,
+        `${clay("#" + cleanName)}  ${versionTag}${replyTag}  ${dim(`(${result.notifications} mentions)`)}`,
       );
       return EXIT.SUCCESS;
     },

apps/cli/src/commands/topic-tail.ts

@@ -27,14 +27,34 @@ export interface TopicTailFlags {
 
 interface TopicMessage {
   id: string;
+  senderMemberId?: string;
   senderPubkey: string;
   senderName: string;
   nonce: string;
   ciphertext: string;
   bodyVersion?: number;
+  replyToId?: string | null;
   createdAt: string;
 }
 
+/** Bounded recent-message cache used to render reply-context lines. */
+type RenderedSnippet = { name: string; snippet: string };
+const RECENT_CACHE_MAX = 256;
+function rememberRendered(
+  cache: Map<string, RenderedSnippet>,
+  m: TopicMessage,
+  text: string,
+): void {
+  cache.set(m.id, {
+    name: m.senderName || m.senderPubkey.slice(0, 8),
+    snippet: text.replace(/\s+/g, " ").slice(0, 60),
+  });
+  if (cache.size > RECENT_CACHE_MAX) {
+    const firstKey = cache.keys().next().value;
+    if (firstKey) cache.delete(firstKey);
+  }
+}
+
 interface HistoryResponse {
   topic: string;
   topicId: string;
@@ -79,16 +99,27 @@ async function printMessage(
   m: TopicMessage,
   topicKey: Uint8Array | null,
   json: boolean,
+  cache: Map<string, RenderedSnippet>,
 ): Promise<void> {
   const text = await decryptForRender(m, topicKey);
   if (json) {
     console.log(JSON.stringify({ ...m, message: text }));
+    rememberRendered(cache, m, text);
     return;
   }
   const v2Marker = (m.bodyVersion ?? 1) === 2 ? dim("🔒 ") : "";
+  if (m.replyToId) {
+    const parent = cache.get(m.replyToId);
+    const ref = parent
+      ? `${parent.name}: "${parent.snippet}${parent.snippet.length === 60 ? "…" : ""}"`
+      : `${m.replyToId.slice(0, 8)}…`;
+    process.stdout.write(`  ${dim("↳ in reply to " + ref)}\n`);
+  }
+  const idTag = dim(`#${m.id.slice(0, 8)}`);
   process.stdout.write(
-    `  ${dim(fmtTime(m.createdAt))}  ${bold(m.senderName || m.senderPubkey.slice(0, 8))}  ${v2Marker}${text}\n`,
+    `  ${dim(fmtTime(m.createdAt))}  ${bold(m.senderName || m.senderPubkey.slice(0, 8))}  ${idTag}  ${v2Marker}${text}\n`,
   );
+  rememberRendered(cache, m, text);
 }
 
 interface SseEvent {
@@ -153,6 +184,7 @@ export async function runTopicTail(name: string, flags: TopicTailFlags): Promise
         topicName: cleanName,
       });
       const topicKey = keyResult.ok ? keyResult.topicKey ?? null : null;
+      const snippetCache = new Map<string, RenderedSnippet>();
 
       // Re-seal background loop. While we hold the topic key, every
       // 30s we look for newly-joined members who don't have a sealed
@@ -241,7 +273,7 @@ export async function runTopicTail(name: string, flags: TopicTailFlags): Promise
           }
           // History is newest-first; reverse for chronological display.
           for (const m of history.messages.slice().reverse()) {
-            await printMessage(m, topicKey, flags.json ?? false);
+            await printMessage(m, topicKey, flags.json ?? false, snippetCache);
           }
         } catch (err) {
           render.warn(`backfill failed: ${(err as Error).message}`);
@@ -283,7 +315,7 @@ export async function runTopicTail(name: string, flags: TopicTailFlags): Promise
           if (ev.event === "message") {
             try {
               const m = JSON.parse(ev.data) as TopicMessage;
-              await printMessage(m, topicKey, flags.json ?? false);
+              await printMessage(m, topicKey, flags.json ?? false, snippetCache);
             } catch {
               // skip malformed
             }