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.

AGENTS.md

@@ -55,22 +55,42 @@ A per-entry `serializer` promise chain prevents concurrent syncs from racing.
 
 ```
 index.ts            — Extension entry point (tools, commands, auto-check flag)
-server.ts           — LSP server registry (gopls, typescript-language-server, pyright)
+server.ts           — Built-in LSP server registry (gopls, typescript-language-server, pyright, ...)
 cli.ts              — CLI for testing/debugging (daemon-aware or --no-daemon)
 daemon.ts           — Entrypoint that starts the daemon process
 
 src/
   client.ts         — LspClient: spawns a language server, JSON-RPC handshake, file sync
   commands.ts       — CLI command dispatcher (maps command names → LSP methods)
+  config.ts         — Per-repo `.pi-lsp.json` loader: walk-up + merge with built-ins, mtime cache
   daemonClient.ts   — High-level helpers (daemonRequest, daemonDiagnostics, etc.)
   daemonProtocol.ts — Shared types, socket path, NDJSON send/receive, autospawn logic
-  root.ts           — pickServer(), findRoot(), URI/path conversion
+  root.ts           — pickServer(), findServerById(), getServersForPath(), findRoot(), URI/path conversion
   types.ts          — ServerConfig interface, LspCommand union
 ```
 
-## Adding a Server
+### Per-Repo Config (`.pi-lsp.json`)
 
-Edit `server.ts`. Add an entry to the `servers` array:
+Users can add/override/disable servers without editing `server.ts`. `src/config.ts`
+walks upward from a given path to find `.pi-lsp.json`, parses it, and merges
+with the built-in `servers` list:
+
+- New `id` → appended (must supply `match`, `command`, `args`, `rootMarkers`).
+- Existing `id` → shallow-merged over the built-in (user fields win).
+- `disable: []` → filtered out at the end.
+
+Results are cached per config path, invalidated by mtime. `getServersForPath(p)`
+is the **single entry point** — don't import the raw `servers` array from
+`server.ts` outside `src/config.ts`. The daemon resolves servers at
+`getOrCreateEntry()` time via `findServerById(filePath, id)`, so spawned
+servers reflect the config of the file being acted on. **Already-running**
+entries don't see config changes; users must `/lsp-destroy` to respawn.
+
+## Adding a Server (Built-In)
+
+For servers shipped with pi-lsp, edit `server.ts`. (For per-repo additions,
+users should drop a `.pi-lsp.json` at the repo root — see README.) Add an entry
+to the `servers` array:
 
 ```typescript
 {