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/paths.ts

@@ -0,0 +1,33 @@
+// Path helpers — single source of truth for where files live.
+//
+// All paths derive from $HOME (or os.homedir() fallback) and the agent's
+// resolved identity. Centralizing them here keeps inbox, hwm, sentinel,
+// and ack-log paths consistent across modules and trivially mockable in
+// tests by overriding `home`.
+
+import { homedir } from "node:os";
+import { join } from "node:path";
+
+export interface Paths {
+  workspace: string;
+  pingsDir: string;
+  acksLog: string;
+  agentsFile: string;
+  inbox(agent: string): string;
+  hwm(agent: string): string;
+  watcherActive(agent: string): string;
+}
+
+export function makePaths(home: string = process.env.HOME ?? homedir()): Paths {
+  const workspace = join(home, "Nyx", "workspace");
+  const pingsDir = join(workspace, "pings");
+  return {
+    workspace,
+    pingsDir,
+    acksLog: join(workspace, "acks", "log"),
+    agentsFile: join(pingsDir, ".agents"),
+    inbox: (agent: string) => join(pingsDir, `${agent}.inbox`),
+    hwm: (agent: string) => join(pingsDir, `.${agent}.hwm`),
+    watcherActive: (agent: string) => join(pingsDir, `.${agent}.watcher-active`),
+  };
+}