feat(watcher): forward FS events as workspace/didChangeWatchedFiles

LSP servers maintain their own workspace index built at initialize time
and rely on the client to push file-system events. Previously the daemon
only synced the single file being queried, so externally created/changed
files (codegen, build scripts, git checkout, the agent's own writes from
the perspective of other open files) left the server's index stale until
manual /lsp-destroy.

Each ClientEntry now lazily owns a WorkspaceWatcher (chokidar + picomatch)
that translates FS events into workspace/didChangeWatchedFiles batches.
Patterns come from the server via client/registerCapability (no
speculative watching). Ignores layer a tiny baseline (.git, .DS_Store)
over the repo's root .gitignore, with a fallback list for non-git
workspaces. Events debounce 50ms quiet / 500ms max wait.

Notable: gopls registers absolute-path globs (/abs/root/**/*.go) rather
than relative ones, so compileWatchers() matches each event against both
relative and absolute path forms. Caught by the integration test; unit
regression test added.

Rollback: PI_LSP_DISABLE_WATCHERS=1 disables all watcher creation.

- src/client.ts: honor register/unregisterCapability for
  workspace/didChangeWatchedFiles; advertise dynamicRegistration;
  expose getFileWatchers/onWatchersChanged/sendNotification
- src/watcher.ts: new WorkspaceWatcher with layered ignores,
  debounce+batch, Created+Deleted coalescing, dual-form glob matching
- src/daemon.ts: per-entry watcher lifecycle, PI_LSP_DISABLE_WATCHERS,
  LSP_DEBUG-gated pattern/event logging
- test/unit/watcher.test.ts: 11 tests against real chokidar + temp dir
- test/integration/watcher-gopls.test.ts: end-to-end against gopls
- AGENTS.md: new "Workspace File Watching" section
- flake.nix: add go (required by gopls integration test)

src/daemon.ts

@@ -8,6 +8,7 @@ import * as path from "node:path";
 import { LspClient } from "./client.ts";
 import { findRoot, findServerById, pathToUri } from "./root.ts";
 import type { ServerConfig } from "./types.ts";
+import { WorkspaceWatcher, type FileEvent } from "./watcher.ts";
 import {
   logPath,
   socketPath,
@@ -37,6 +38,11 @@ interface ClientEntry {
   idleTimer: NodeJS.Timeout | null;
   ttlMs: number;
   lastUsed: number;
+  // watcher: Lazy - created on first registerCapability for watched files,
+  // disposed on eviction. Null if the server never registered any watchers
+  // (or if PI_LSP_DISABLE_WATCHERS is set).
+  watcher: WorkspaceWatcher | null;
+  unsubscribeWatchers: (() => void) | null;
 }
 
 const entries = new Map<string, ClientEntry>();
@@ -88,6 +94,8 @@ async function getOrCreateEntry(
     idleTimer: null,
     ttlMs,
     lastUsed: Date.now(),
+    watcher: null,
+    unsubscribeWatchers: null,
   };
   entries.set(key, entry);
   try {
@@ -96,10 +104,52 @@ async function getOrCreateEntry(
     entries.delete(key);
     throw err;
   }
+  attachWatcher(entry);
   bumpIdle(entry);
   return entry;
 }
 
+// Attach Watcher - Subscribes to the client's watcher-registration events.
+// The first non-empty registration lazily creates the WorkspaceWatcher;
+// subsequent register/unregister calls update its pattern set in place.
+// Honors PI_LSP_DISABLE_WATCHERS for emergency rollback.
+function attachWatcher(entry: ClientEntry): void {
+  if (process.env.PI_LSP_DISABLE_WATCHERS) return;
+  const sync = () => {
+    const patterns = entry.client.getFileWatchers();
+    if (patterns.length === 0 && !entry.watcher) return;
+    if (!entry.watcher) {
+      entry.watcher = new WorkspaceWatcher(entry.rootDir, (events) =>
+        forwardEvents(entry, events),
+      );
+      log(`watcher`, entry.server.id, entry.rootDir, `patterns=${patterns.length}`);
+    }
+    if (process.env.LSP_DEBUG) {
+      log(`watcher patterns`, entry.server.id, JSON.stringify(patterns));
+    }
+    entry.watcher.setPatterns(patterns);
+  };
+  entry.unsubscribeWatchers = entry.client.onWatchersChanged(sync);
+  // Initial Sync - Server may have already sent registerCapability during
+  // initialize before we subscribed.
+  sync();
+}
+
+// Forward Events - Sends a batched workspace/didChangeWatchedFiles to the
+// server. We catch errors so a downed transport doesn't crash the daemon.
+function forwardEvents(entry: ClientEntry, events: FileEvent[]): void {
+  try {
+    if (process.env.LSP_DEBUG) {
+      log(`watcher fire`, entry.server.id, JSON.stringify(events));
+    }
+    entry.client.sendNotification("workspace/didChangeWatchedFiles", {
+      changes: events,
+    });
+  } catch (err) {
+    log(`watcher send failed`, entry.server.id, (err as Error).message);
+  }
+}
+
 // Bump Idle - Resets the idle eviction timer. Called on every request that
 // touches the entry. We log evictions so the daemon's behavior is visible.
 function bumpIdle(entry: ClientEntry) {
@@ -113,6 +163,8 @@ function evict(entry: ClientEntry, reason: string) {
   log(`evict`, entry.key, reason);
   entries.delete(entry.key);
   if (entry.idleTimer) clearTimeout(entry.idleTimer);
+  if (entry.unsubscribeWatchers) entry.unsubscribeWatchers();
+  void entry.watcher?.dispose();
   void entry.client.dispose();
   // Auto Shutdown - If this was the last entry, there's nothing left to
   // manage. Tear down the daemon so it doesn't sit idle forever.
@@ -292,6 +344,8 @@ function shutdownDaemon(reason: string) {
   if (server) server.close();
   for (const entry of entries.values()) {
     if (entry.idleTimer) clearTimeout(entry.idleTimer);
+    if (entry.unsubscribeWatchers) entry.unsubscribeWatchers();
+    void entry.watcher?.dispose();
     void entry.client.dispose();
   }
   entries.clear();