Layer 2 MCP Watcher v0 scaffold

Per spec/agent-watcher.md §4. TypeScript/Node implementation living in
mcp-watcher/ subdirectory, parallel to Layer 1 Collector at repo root.

What lands:
- Core MCP server (src/server.ts) with experimental['claude/channel']:{}
  + tools:{} capability declarations, stdio transport, channel-event
  notifier wired through the inbox watcher.
- Identity resolution mirroring agent-ping's layered model
  (PING_AGENT_IDENTITY env, $CLAUDE_HOME/ping-agent, ~/.ping-agent).
- Inbox reader with HWM tracking, sentinel deferral (warn-after-3),
  atomic HWM writes via tmp+rename.
- chokidar-backed file watcher with coalesced drain, urgent-first
  ordering, recentEvents map for tool sender lookup.
- Three reply tools (ack / respond / mark_handled) with cross-host
  write discipline (writes to local inbox files; Syncthing replicates).
- Sentinel file (.<agent>.watcher-active) for hook coexistence per
  spec §4.3 — agent-ping hook stands down when the watcher is in
  charge of delivery on this host. Sentinel + hwm in .stignore.
- 35 unit tests passing (vitest): inbox parsing, HWM round-trip,
  sentinel deferral semantics, identity layers, tool I/O, watcher
  drain + ordering + restart-from-hwm.
- install.sh (Angus-executed, rule-2 compliant) installs deps,
  builds, symlinks ~/.local/bin/agent-watcher-mcp, prints mcp.json
  registration snippet for paste.
- README documents launch flag, sandbox CLAUDE_HOME pattern,
  hook coexistence, observability, v2 limitations.

Not yet:
- Integration test against a real Claude Code session — gated on
  Angus spinning up a sandbox CC session on the VPS with
  CLAUDE_HOME=~/.claude-sandbox.
- agent-ping hook update to read the sentinel and stand down.
  Separate small PR against agent-ping.

Interface contract with Layer 1: the inbox JSONL line shape from
inbox.ts::PingEvent matches inbox.Event in the Collector — bit-
identical reads regardless of source.

mcp-watcher/src/identity.ts

@@ -0,0 +1,57 @@
+// Identity resolution for the MCP Watcher.
+//
+// Mirrors the layered resolution shipped in agent-ping (PR #1). The CLI
+// has four layers (--as flag, env, $CLAUDE_HOME/ping-agent, ~/.ping-agent);
+// the watcher subprocess has three (no argv override useful in MCP context).
+//
+// Spec: agent-ping spec §9; agent-watcher spec §4.4.
+
+import { homedir } from "node:os";
+import { existsSync, readFileSync } from "node:fs";
+import { join } from "node:path";
+
+export interface ResolvedIdentity {
+  agent: string;
+  source: string;
+}
+
+/**
+ * Resolve this process's agent identity.
+ * Layers, first match wins:
+ *   1. PING_AGENT_IDENTITY env var
+ *   2. $CLAUDE_HOME/ping-agent (if CLAUDE_HOME is set and file exists)
+ *   3. ~/.ping-agent
+ *
+ * Throws if no layer resolves.
+ */
+export function resolveIdentity(env: NodeJS.ProcessEnv = process.env): ResolvedIdentity {
+  const envId = (env.PING_AGENT_IDENTITY ?? "").trim();
+  if (envId) {
+    return { agent: envId, source: "PING_AGENT_IDENTITY env var" };
+  }
+
+  const claudeHome = (env.CLAUDE_HOME ?? "").trim();
+  if (claudeHome) {
+    const f = join(claudeHome, "ping-agent");
+    if (existsSync(f)) {
+      const name = readFileSync(f, "utf8").trim();
+      if (name) {
+        return { agent: name, source: f };
+      }
+    }
+  }
+
+  const home = env.HOME ?? homedir();
+  const defaultFile = join(home, ".ping-agent");
+  if (existsSync(defaultFile)) {
+    const name = readFileSync(defaultFile, "utf8").trim();
+    if (name) {
+      return { agent: name, source: defaultFile };
+    }
+  }
+
+  throw new Error(
+    `agent-watcher-mcp: no identity resolved. Set PING_AGENT_IDENTITY, ` +
+      `$CLAUDE_HOME/ping-agent, or ${defaultFile}.`,
+  );
+}