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.

src/daemon.ts

@@ -6,9 +6,8 @@ import * as fs from "node:fs";
 import * as net from "node:net";
 import * as path from "node:path";
 import { LspClient } from "./client.ts";
-import { findRoot, pathToUri } from "./root.ts";
+import { findRoot, findServerById, pathToUri } from "./root.ts";
 import type { ServerConfig } from "./types.ts";
-import { servers } from "../server.ts";
 import {
   logPath,
   socketPath,
@@ -51,25 +50,17 @@ function log(...args: unknown[]) {
   );
 }
 
-// Find Server By ID - Looks up a ServerConfig from the registry by ID.
-// Throws if the ID is not registered.
-function findServerById(serverId: string): ServerConfig {
-  const server = servers.find((s) => s.id === serverId);
-  if (!server) {
-    throw new Error(`Unknown server ID: "${serverId}". Registered: ${servers.map((s) => s.id).join(", ")}`);
-  }
-  return server;
-}
-
 // Get Or Create Entry - Looks up the cached client for a server+file,
 // spawning a fresh LspClient if needed. The returned entry is guaranteed
-// to have its `ready` promise resolved before the caller uses it.
+// to have its `ready` promise resolved before the caller uses it. The
+// server registry is resolved against any `.pi-lsp.json` reachable from
+// `filePath`, so per-repo config overrides take effect at spawn time.
 async function getOrCreateEntry(
   filePath: string,
   serverId: string,
   launch: LaunchContext,
 ): Promise<ClientEntry> {
-  const server = findServerById(serverId);
+  const server = findServerById(filePath, serverId);
   const rootDir = findRoot(filePath, server.rootMarkers);
   const key = `${server.id}::${rootDir}`;
   const existing = entries.get(key);