feat(config): add per-repo .pi-lsp.json server overrides

Users can now drop a .pi-lsp.json at any ancestor of their working
files to add new LSP servers, override built-in ones, or disable
servers entirely. The nearest config (walking upward) wins.

- New src/config.ts: walks upward for .pi-lsp.json, parses, and
  merges with the built-in registry. Cached per config-file path
  with mtime invalidation. Falls back to built-ins on parse error.
- Merge rules: matching id shallow-merges (user wins); new id
  appends (must include match/command/args/rootMarkers); `disable`
  filters at the end.
- src/root.ts: pickServer() now resolves servers via the per-repo
  registry. Adds findServerById(filePath, id) and re-exports
  getServersForPath() for callers.
- src/daemon.ts: getOrCreateEntry() resolves serverId against the
  filePath's config so spawned servers reflect repo overrides.
- index.ts and cli.ts: replace direct `servers` imports with
  path-aware getServersForPath() lookups.
- Tests: 9 new unit tests covering merge semantics, walk-up
  discovery, mtime invalidation, and graceful fallback.
- Docs: README "Per-Repo Config" section + AGENTS.md updates.

README.md

@@ -123,6 +123,44 @@ Edit `server.ts`:
 }
 ```
 
+## Per-Repo Config (`.pi-lsp.json`)
+
+Drop a `.pi-lsp.json` at any ancestor of your working files to add or override
+servers for that repo. The nearest config (walking upward) wins.
+
+```json
+{
+  "servers": [
+    {
+      "id": "rust-analyzer",
+      "match": ["rs"],
+      "command": "rust-analyzer",
+      "args": [],
+      "rootMarkers": ["Cargo.toml"],
+      "languageId": "rust"
+    },
+    { "id": "gopls", "args": ["-remote=auto", "-vv"] }
+  ],
+  "disable": ["oxlint"]
+}
+```
+
+**Merge rules:**
+
+- Entry with a built-in `id` → fields shallow-merge over the built-in (user wins).
+- Entry with a new `id` → appended; must include `match`, `command`, `args`, `rootMarkers`.
+- `disable` → filters out matching ids (built-in or user-defined).
+
+**Reloading after edits:** Config is re-read on mtime change, so new lookups
+pick up changes automatically. However, **already-running** language servers in
+the daemon keep their original spawn args. If you change `command`, `args`, or
+`rootMarkers`, run `/lsp-destroy` (or `pi-lsp daemon stop`) so they respawn
+with the new config.
+
+**Security note:** `.pi-lsp.json` controls what binary pi-lsp spawns. Treat it
+like `.vscode/settings.json` — don't accept untrusted configs from arbitrary
+repos.
+
 ## Adding A Command
 
 1. Add to the `LspCommand` union in `src/types.ts`.