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)

package-lock.json

@@ -8,6 +8,9 @@
       "name": "@evan/pi-lsp",
       "version": "0.1.0",
       "dependencies": {
+        "chokidar": "^5.0.0",
+        "ignore": "^7.0.5",
+        "picomatch": "^4.0.4",
         "vscode-jsonrpc": "^8.2.1",
         "vscode-languageserver-protocol": "^3.17.5"
       },
@@ -17,6 +20,7 @@
       "devDependencies": {
         "@mariozechner/pi-coding-agent": "^0.72.0",
         "@types/node": "^22.10.0",
+        "@types/picomatch": "^4.0.3",
         "oxlint": "^1.62.0",
         "tsx": "^4.19.2",
         "typebox": "^1.1.37",
@@ -2789,6 +2793,13 @@
         "undici-types": "~6.21.0"
       }
     },
+    "node_modules/@types/picomatch": {
+      "version": "4.0.3",
+      "resolved": "https://registry.npmjs.org/@types/picomatch/-/picomatch-4.0.3.tgz",
+      "integrity": "sha512-iG0T6+nYJ9FAPmx9SsUlnwcq1ZVRuCXcVEvWnntoPlrOpwtSTKNDC9uVAxTsC3PUvJ+99n4RpAcNgBbHX3JSnQ==",
+      "dev": true,
+      "license": "MIT"
+    },
     "node_modules/@types/retry": {
       "version": "0.12.0",
       "resolved": "https://registry.npmjs.org/@types/retry/-/retry-0.12.0.tgz",
@@ -2967,6 +2978,21 @@
         "url": "https://github.com/chalk/chalk?sponsor=1"
       }
     },
+    "node_modules/chokidar": {
+      "version": "5.0.0",
+      "resolved": "https://registry.npmjs.org/chokidar/-/chokidar-5.0.0.tgz",
+      "integrity": "sha512-TQMmc3w+5AxjpL8iIiwebF73dRDF4fBIieAqGn9RGCWaEVwQ6Fb2cGe31Yns0RRIzii5goJ1Y7xbMwo1TxMplw==",
+      "license": "MIT",
+      "dependencies": {
+        "readdirp": "^5.0.0"
+      },
+      "engines": {
+        "node": ">= 20.19.0"
+      },
+      "funding": {
+        "url": "https://paulmillr.com/funding/"
+      }
+    },
     "node_modules/cli-highlight": {
       "version": "2.1.11",
       "resolved": "https://registry.npmjs.org/cli-highlight/-/cli-highlight-2.1.11.tgz",
@@ -3642,7 +3668,6 @@
       "version": "7.0.5",
       "resolved": "https://registry.npmjs.org/ignore/-/ignore-7.0.5.tgz",
       "integrity": "sha512-Hs59xBNfUIunMFgWAbGX5cq6893IbWg4KnrjbYwX3tx0ztorVgTDA6B2sxf8ejHJ4wz8BqGUMYlnzNBer5NvGg==",
-      "dev": true,
       "license": "MIT",
       "engines": {
         "node": ">= 4"
@@ -4085,6 +4110,18 @@
       "dev": true,
       "license": "MIT"
     },
+    "node_modules/picomatch": {
+      "version": "4.0.4",
+      "resolved": "https://registry.npmjs.org/picomatch/-/picomatch-4.0.4.tgz",
+      "integrity": "sha512-QP88BAKvMam/3NxH6vj2o21R6MjxZUAd6nlwAS/pnGvN9IVLocLHxGYIzFhg6fUQ+5th6P4dv4eW9jX3DSIj7A==",
+      "license": "MIT",
+      "engines": {
+        "node": ">=12"
+      },
+      "funding": {
+        "url": "https://github.com/sponsors/jonschlinkert"
+      }
+    },
     "node_modules/proper-lockfile": {
       "version": "4.1.2",
       "resolved": "https://registry.npmjs.org/proper-lockfile/-/proper-lockfile-4.1.2.tgz",
@@ -4180,6 +4217,19 @@
         "once": "^1.3.1"
       }
     },
+    "node_modules/readdirp": {
+      "version": "5.0.0",
+      "resolved": "https://registry.npmjs.org/readdirp/-/readdirp-5.0.0.tgz",
+      "integrity": "sha512-9u/XQ1pvrQtYyMpZe7DXKv2p5CNvyVwzUB6uhLAnQwHMSgKMBR62lc7AHljaeteeHXn11XTAaLLUVZYVZyuRBQ==",
+      "license": "MIT",
+      "engines": {
+        "node": ">= 20.19.0"
+      },
+      "funding": {
+        "type": "individual",
+        "url": "https://paulmillr.com/funding/"
+      }
+    },
     "node_modules/require-directory": {
       "version": "2.1.1",
       "resolved": "https://registry.npmjs.org/require-directory/-/require-directory-2.1.1.tgz",