feat(plugin): per-session mailbox identity + mailbox-update command

The hook now derives a unique mailbox name from the session_id supplied
on hook stdin, so two parallel Claude Code sessions in the same project
get distinct mailboxes (e.g. `claude-a8b3c1d2`, `claude-d4e5f6a7`)
instead of colliding on a shared env value. An optional
CLAUDE_MAILBOX_NAME base prefix flavors the names as `<base>-<sid>`.

Adds:
- `claude-mailbox session-announce` subcommand for the new SessionStart
  hook, which prints the current session's mailbox name to context
- `/claude-mailbox:mailbox-update` slash command for `npm update` +
  daemon restart
- stdin parsing helpers (parseHookStdin, deriveSessionName) with unit
  tests; the doctor no longer needs a mandatory name prompt

node/src/cli.ts

@@ -10,8 +10,11 @@ import {
   applyInstall,
   applyUninstall,
   buildHookCommand,
+  deriveSessionName,
   formatMessagesForHook,
+  parseHookStdin,
   readSettings,
+  readStdinIfPiped,
   settingsPathFor,
   writeSettings,
   type HookMessage,
@@ -125,19 +128,36 @@ program
     }
   });
 
+function resolveHookMailboxName(explicit: string | undefined): string | null {
+  if (explicit && explicit.trim()) return explicit.trim();
+  const stdin = parseHookStdin(readStdinIfPiped());
+  const sid = stdin?.session_id?.trim();
+  if (sid) {
+    const base = (process.env["CLAUDE_MAILBOX_NAME"] ?? "").trim() || null;
+    return deriveSessionName(sid, base);
+  }
+  const envName = (process.env["CLAUDE_MAILBOX_NAME"] ?? "").trim();
+  return envName || null;
+}
+
 program
   .command("check")
   .description(
-    "Pull pending messages and mark delivered. In --hook mode the name can come from CLAUDE_MAILBOX_NAME.",
+    "Pull pending messages and mark delivered. In --hook mode the name is auto-derived from the SessionStart/UserPromptSubmit stdin (session_id), optionally flavored by $CLAUDE_MAILBOX_NAME.",
+  )
+  .option(
+    "--name <name>",
+    "Explicit mailbox name. Overrides hook stdin and $CLAUDE_MAILBOX_NAME.",
   )
-  .option("--name <name>", "Mailbox name (also sent as X-Mailbox). Falls back to $CLAUDE_MAILBOX_NAME.")
   .option("--url <url>", "Daemon base URL", DEFAULT_URL)
   .option(
     "--hook",
-    "Hook mode: human-readable output, silent when no mailbox configured or daemon unreachable (exit 0). Emits a one-line setup hint when name is set but daemon is unreachable.",
+    "Hook mode: human-readable output, silent when no mailbox configured or daemon unreachable. Emits a one-line setup hint when name resolves but daemon is unreachable.",
   )
   .action(async (opts: { name?: string; url: string; hook?: boolean }) => {
-    const name = (opts.name ?? process.env["CLAUDE_MAILBOX_NAME"] ?? "").trim();
+    const name = opts.hook
+      ? resolveHookMailboxName(opts.name)
+      : (opts.name ?? process.env["CLAUDE_MAILBOX_NAME"] ?? "").trim() || null;
     if (!name) {
       if (opts.hook) return;
       console.error("Missing --name (or set CLAUDE_MAILBOX_NAME).");
@@ -170,6 +190,23 @@ program
     }
   });
 
+program
+  .command("session-announce")
+  .description(
+    "SessionStart-hook helper: reads the hook stdin JSON, derives the session's mailbox name from session_id, and prints a one-line announcement to stdout so Claude sees its mailbox identity.",
+  )
+  .action(() => {
+    const stdin = parseHookStdin(readStdinIfPiped());
+    const sid = stdin?.session_id?.trim();
+    if (!sid) return;
+    const base = (process.env["CLAUDE_MAILBOX_NAME"] ?? "").trim() || null;
+    const name = deriveSessionName(sid, base);
+    process.stdout.write(
+      `Claude-Mailbox: this session is mailbox \`${name}\`. ` +
+        `Peers can send to it with: claude-mailbox send --from <peer> --to ${name} --body "..."\n`,
+    );
+  });
+
 program
   .command("list")
   .description("List known mailboxes.")

node/src/hook.ts

@@ -2,6 +2,45 @@ import { existsSync, mkdirSync, readFileSync, writeFileSync } from "node:fs";
 import { homedir } from "node:os";
 import { dirname, join } from "node:path";
 
+export interface HookStdinPayload {
+  session_id?: string;
+  hook_event_name?: string;
+  [key: string]: unknown;
+}
+
+export function parseHookStdin(raw: string | null | undefined): HookStdinPayload | null {
+  if (!raw || !raw.trim()) return null;
+  try {
+    const parsed = JSON.parse(raw) as unknown;
+    if (parsed && typeof parsed === "object") return parsed as HookStdinPayload;
+    return null;
+  } catch {
+    return null;
+  }
+}
+
+export function readStdinIfPiped(): string | null {
+  if (process.stdin.isTTY) return null;
+  try {
+    return readFileSync(0, "utf8");
+  } catch {
+    return null;
+  }
+}
+
+export function shortSessionId(sessionId: string): string {
+  const hex = sessionId.replace(/[^0-9a-fA-F]/g, "").toLowerCase();
+  if (hex.length >= 8) return hex.slice(0, 8);
+  return sessionId.toLowerCase().replace(/[^a-z0-9]/g, "").slice(0, 8) || "00000000";
+}
+
+export function deriveSessionName(sessionId: string, base?: string | null): string {
+  const short = shortSessionId(sessionId);
+  const trimmed = (base ?? "").trim();
+  if (trimmed) return `${trimmed}-${short}`;
+  return `claude-${short}`;
+}
+
 export interface HookMessage {
   id: number;
   from: string;