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/config.ts

@@ -0,0 +1,141 @@
+// Per-Repo Config - Loads `.pi-lsp.json` from the nearest ancestor of a
+// given path and merges it with the built-in server registry. Config is a
+// JSON file shaped like:
+//
+//   {
+//     "servers": [
+//       { "id": "rust-analyzer", "match": ["rs"], "command": "rust-analyzer",
+//         "args": [], "rootMarkers": ["Cargo.toml"], "languageId": "rust" },
+//       { "id": "gopls", "args": ["-remote=auto", "-vv"] }
+//     ],
+//     "disable": ["oxlint"]
+//   }
+//
+// Merge Semantics:
+//   - Entries whose `id` matches a built-in shallow-merge their fields (user wins).
+//   - Entries with a new `id` append, and must include all required fields.
+//   - The `disable` list filters the merged result by id.
+//
+// Caching: Resolved lists are cached per config-file path and invalidated by
+// mtime. If no config file exists, the built-in registry is returned.
+import * as fs from "node:fs";
+import * as path from "node:path";
+import { servers as builtinServers } from "../server.ts";
+import type { ServerConfig } from "./types.ts";
+
+const CONFIG_FILE = ".pi-lsp.json";
+
+interface PiLspConfig {
+  servers?: Array<Partial<ServerConfig> & { id: string }>;
+  disable?: string[];
+}
+
+interface CacheEntry {
+  mtimeMs: number;
+  resolved: ServerConfig[];
+}
+
+const cache = new Map<string, CacheEntry>();
+
+// Find Config File - Walks upward from `fromDir` looking for `.pi-lsp.json`.
+// Returns the absolute path or null if none found before the filesystem root.
+function findConfigFile(fromDir: string): string | null {
+  let dir = path.resolve(fromDir);
+  const { root } = path.parse(dir);
+  while (true) {
+    const candidate = path.join(dir, CONFIG_FILE);
+    if (fs.existsSync(candidate)) return candidate;
+    if (dir === root) return null;
+    dir = path.dirname(dir);
+  }
+}
+
+// Merge Config - Built-ins by id, user overrides shallow-merge in, new ids
+// append (with required-field validation), and `disable` filters the result.
+function mergeConfig(config: PiLspConfig, sourcePath: string): ServerConfig[] {
+  const byId = new Map<string, ServerConfig>(
+    builtinServers.map((s) => [s.id, { ...s }]),
+  );
+
+  // Apply Overrides And New Servers
+  for (const override of config.servers ?? []) {
+    if (!override.id || typeof override.id !== "string") {
+      throw new Error(
+        `pi-lsp config (${sourcePath}): every entry in "servers" must have a string "id"`,
+      );
+    }
+    const existing = byId.get(override.id);
+    if (existing) {
+      byId.set(override.id, { ...existing, ...override } as ServerConfig);
+    } else {
+      // New Server - Must include all required spawn fields.
+      const required = ["match", "command", "args", "rootMarkers"] as const;
+      const missing = required.filter((k) => override[k] === undefined);
+      if (missing.length > 0) {
+        throw new Error(
+          `pi-lsp config (${sourcePath}): new server "${override.id}" is missing required field(s): ${missing.join(", ")}`,
+        );
+      }
+      byId.set(override.id, override as ServerConfig);
+    }
+  }
+
+  // Apply Disable Filter
+  const disabled = new Set(config.disable ?? []);
+  return Array.from(byId.values()).filter((s) => !disabled.has(s.id));
+}
+
+// Get Servers For Path - Returns the merged ServerConfig list applicable to
+// `fromDir` (or the directory containing a file path). Cached per config-file
+// path with mtime invalidation. On parse/merge failure, logs to stderr and
+// falls back to the built-in registry so a broken config never breaks LSP.
+export function getServersForPath(fromPath: string): ServerConfig[] {
+  // Resolve To Directory - Accept either a file path or a directory.
+  let dir = path.resolve(fromPath);
+  try {
+    const st = fs.statSync(dir);
+    if (!st.isDirectory()) dir = path.dirname(dir);
+  } catch {
+    dir = path.dirname(dir);
+  }
+
+  const configPath = findConfigFile(dir);
+  if (!configPath) return builtinServers;
+
+  let mtimeMs: number;
+  try {
+    mtimeMs = fs.statSync(configPath).mtimeMs;
+  } catch {
+    return builtinServers;
+  }
+
+  const cached = cache.get(configPath);
+  if (cached && cached.mtimeMs === mtimeMs) return cached.resolved;
+
+  let parsed: PiLspConfig;
+  try {
+    const raw = fs.readFileSync(configPath, "utf8");
+    parsed = JSON.parse(raw);
+  } catch (err) {
+    process.stderr.write(
+      `pi-lsp: failed to read/parse ${configPath}: ${(err as Error).message}\n`,
+    );
+    return builtinServers;
+  }
+
+  let resolved: ServerConfig[];
+  try {
+    resolved = mergeConfig(parsed, configPath);
+  } catch (err) {
+    process.stderr.write(`pi-lsp: ${(err as Error).message}\n`);
+    return builtinServers;
+  }
+
+  cache.set(configPath, { mtimeMs, resolved });
+  return resolved;
+}
+
+// Clear Config Cache - Test hook; not used in production.
+export function clearConfigCache(): void {
+  cache.clear();
+}