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/tests/hook.test.ts

@@ -6,8 +6,11 @@ import {
   applyInstall,
   applyUninstall,
   buildHookCommand,
+  deriveSessionName,
   formatMessagesForHook,
+  parseHookStdin,
   readSettings,
+  shortSessionId,
   writeSettings,
 } from "../src/hook.js";
 
@@ -169,6 +172,73 @@ describe("applyUninstall", () => {
   });
 });
 
+describe("parseHookStdin", () => {
+  it("returns null for empty or whitespace input", () => {
+    expect(parseHookStdin(null)).toBeNull();
+    expect(parseHookStdin("")).toBeNull();
+    expect(parseHookStdin("   \n  ")).toBeNull();
+  });
+
+  it("returns null for non-JSON input", () => {
+    expect(parseHookStdin("not json")).toBeNull();
+    expect(parseHookStdin("{")).toBeNull();
+  });
+
+  it("returns null for JSON primitives (only objects allowed)", () => {
+    expect(parseHookStdin("42")).toBeNull();
+    expect(parseHookStdin("\"foo\"")).toBeNull();
+    expect(parseHookStdin("null")).toBeNull();
+  });
+
+  it("parses a hook payload", () => {
+    const out = parseHookStdin(
+      JSON.stringify({
+        session_id: "abc12345-de67-89f0-1234-567890abcdef",
+        hook_event_name: "UserPromptSubmit",
+        prompt: "hi",
+      }),
+    );
+    expect(out?.session_id).toBe("abc12345-de67-89f0-1234-567890abcdef");
+    expect(out?.hook_event_name).toBe("UserPromptSubmit");
+  });
+});
+
+describe("shortSessionId / deriveSessionName", () => {
+  it("takes first 8 hex chars from a UUID", () => {
+    expect(shortSessionId("abc12345-de67-89f0-1234-567890abcdef")).toBe("abc12345");
+  });
+
+  it("normalizes case and ignores hyphens", () => {
+    expect(shortSessionId("ABC12345-DE67-89F0-1234-567890ABCDEF")).toBe("abc12345");
+  });
+
+  it("falls back to a sanitized prefix for non-hex ids", () => {
+    expect(shortSessionId("session-Test123")).toBe("sessiont");
+  });
+
+  it("derives anonymous name when no base", () => {
+    expect(deriveSessionName("abc12345-de67-89f0-1234-567890abcdef")).toBe("claude-abc12345");
+  });
+
+  it("prepends base prefix when given", () => {
+    expect(deriveSessionName("abc12345-de67-89f0-1234-567890abcdef", "backend")).toBe(
+      "backend-abc12345",
+    );
+  });
+
+  it("treats whitespace-only base as no base", () => {
+    expect(deriveSessionName("abc12345-de67-89f0-1234-567890abcdef", "  ")).toBe(
+      "claude-abc12345",
+    );
+  });
+
+  it("derives different names for different sessions with the same base", () => {
+    const a = deriveSessionName("aaaa1111-de67-89f0-1234-567890abcdef", "shared");
+    const b = deriveSessionName("bbbb2222-de67-89f0-1234-567890abcdef", "shared");
+    expect(a).not.toBe(b);
+  });
+});
+
 describe("readSettings / writeSettings roundtrip", () => {
   it("survives an install → write → read cycle", () => {
     const dir = mkdtempSync(join(tmpdir(), "claude-mailbox-hook-"));