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.

test/unit/config.test.ts

@@ -0,0 +1,130 @@
+// Config Unit Tests — getServersForPath() merge semantics, mtime caching,
+// and graceful fallback on parse errors.
+import { describe, it, beforeEach, afterEach } from "node:test";
+import * as assert from "node:assert/strict";
+import * as fs from "node:fs";
+import * as os from "node:os";
+import * as path from "node:path";
+import { getServersForPath, clearConfigCache } from "../../src/config.ts";
+import { servers as builtinServers } from "../../server.ts";
+
+let tmpDir: string;
+
+beforeEach(() => {
+  tmpDir = fs.mkdtempSync(path.join(os.tmpdir(), "pi-lsp-config-test-"));
+  clearConfigCache();
+});
+
+afterEach(() => {
+  fs.rmSync(tmpDir, { recursive: true, force: true });
+  clearConfigCache();
+});
+
+function writeConfig(dir: string, content: unknown): void {
+  fs.writeFileSync(path.join(dir, ".pi-lsp.json"), JSON.stringify(content));
+}
+
+describe("getServersForPath", () => {
+  it("returns built-in servers when no config file is found", () => {
+    // tmpDir lives under /tmp; no ancestor will have .pi-lsp.json.
+    const result = getServersForPath(tmpDir);
+    assert.deepStrictEqual(
+      result.map((s) => s.id),
+      builtinServers.map((s) => s.id),
+    );
+  });
+
+  it("appends a new server entry from config", () => {
+    writeConfig(tmpDir, {
+      servers: [
+        {
+          id: "rust-analyzer",
+          match: ["rs"],
+          command: "rust-analyzer",
+          args: [],
+          rootMarkers: ["Cargo.toml"],
+          languageId: "rust",
+        },
+      ],
+    });
+    const result = getServersForPath(tmpDir);
+    const rust = result.find((s) => s.id === "rust-analyzer");
+    assert.ok(rust, "rust-analyzer should be present");
+    assert.deepStrictEqual(rust!.match, ["rs"]);
+    // Built-ins are still there
+    assert.ok(result.find((s) => s.id === "gopls"));
+  });
+
+  it("shallow-merges overrides into existing built-in servers", () => {
+    writeConfig(tmpDir, {
+      servers: [{ id: "gopls", args: ["-vv"] }],
+    });
+    const result = getServersForPath(tmpDir);
+    const gopls = result.find((s) => s.id === "gopls")!;
+    assert.deepStrictEqual(gopls.args, ["-vv"]);
+    // Other fields preserved from built-in
+    assert.strictEqual(gopls.command, "gopls");
+    assert.deepStrictEqual(gopls.match, ["go"]);
+  });
+
+  it("filters out servers listed in `disable`", () => {
+    writeConfig(tmpDir, { disable: ["oxlint", "gopls"] });
+    const result = getServersForPath(tmpDir);
+    assert.ok(!result.some((s) => s.id === "oxlint"));
+    assert.ok(!result.some((s) => s.id === "gopls"));
+    assert.ok(result.some((s) => s.id === "pyright"));
+  });
+
+  it("walks upward to find config in an ancestor directory", () => {
+    writeConfig(tmpDir, { disable: ["oxlint"] });
+    const sub = path.join(tmpDir, "a", "b", "c");
+    fs.mkdirSync(sub, { recursive: true });
+    const result = getServersForPath(sub);
+    assert.ok(!result.some((s) => s.id === "oxlint"));
+  });
+
+  it("accepts a file path and walks from its directory", () => {
+    writeConfig(tmpDir, { disable: ["oxlint"] });
+    const file = path.join(tmpDir, "src", "main.go");
+    fs.mkdirSync(path.dirname(file), { recursive: true });
+    fs.writeFileSync(file, "package main");
+    const result = getServersForPath(file);
+    assert.ok(!result.some((s) => s.id === "oxlint"));
+  });
+
+  it("falls back to built-ins on invalid JSON", () => {
+    fs.writeFileSync(path.join(tmpDir, ".pi-lsp.json"), "{ not valid");
+    const result = getServersForPath(tmpDir);
+    assert.deepStrictEqual(
+      result.map((s) => s.id),
+      builtinServers.map((s) => s.id),
+    );
+  });
+
+  it("falls back to built-ins when a new server is missing required fields", () => {
+    writeConfig(tmpDir, {
+      servers: [{ id: "incomplete", match: ["foo"] }],
+    });
+    const result = getServersForPath(tmpDir);
+    assert.deepStrictEqual(
+      result.map((s) => s.id),
+      builtinServers.map((s) => s.id),
+    );
+  });
+
+  it("invalidates cache on mtime change", () => {
+    writeConfig(tmpDir, { disable: ["oxlint"] });
+    let result = getServersForPath(tmpDir);
+    assert.ok(!result.some((s) => s.id === "oxlint"));
+
+    // Rewrite With Different Content And Bump mtime
+    const cfgPath = path.join(tmpDir, ".pi-lsp.json");
+    fs.writeFileSync(cfgPath, JSON.stringify({ disable: ["gopls"] }));
+    const future = new Date(Date.now() + 5000);
+    fs.utimesSync(cfgPath, future, future);
+
+    result = getServersForPath(tmpDir);
+    assert.ok(result.some((s) => s.id === "oxlint"));
+    assert.ok(!result.some((s) => s.id === "gopls"));
+  });
+});