perf(rules-injector): cache project root for visited ancestors

findProjectRoot was keyed by exact startPath, so sibling files in the
same project repeated the entire upward marker walk. The walk does one
existsSync per marker per ancestor directory, which adds up on every
read/write/edit/multiedit tool call.

Track every directory visited during the walk and seed the cache with
the resolved root for each of them. Subsequent lookups for any
descendant short-circuit to the cached ancestor without re-running
marker probes. Cache invalidation still happens on session.deleted /
session.compacted, so production semantics are unchanged.

Pin the new contract via a sibling-startpath test, and make the
existing finder.test.ts beforeEach explicit about cache state so the
more aggressive cache does not leak between tests.

src/hooks/rules-injector/finder.test.ts

@@ -3,12 +3,14 @@ import { existsSync, mkdirSync, rmSync, writeFileSync } from "node:fs";
 import { tmpdir } from "node:os";
 import { join } from "node:path";
 import { findProjectRoot, findRuleFiles } from "./finder";
+import { clearProjectRootCache } from "./project-root-finder";
 
 describe("findRuleFiles", () => {
   const TEST_DIR = join(tmpdir(), `rules-injector-test-${Date.now()}`);
   const homeDir = join(TEST_DIR, "home");
 
   beforeEach(() => {
+    clearProjectRootCache();
     mkdirSync(TEST_DIR, { recursive: true });
     mkdirSync(homeDir, { recursive: true });
     mkdirSync(join(TEST_DIR, ".git"), { recursive: true });
@@ -328,6 +330,7 @@ describe("findProjectRoot", () => {
   const TEST_DIR = join(tmpdir(), `project-root-test-${Date.now()}`);
 
   beforeEach(() => {
+    clearProjectRootCache();
     mkdirSync(TEST_DIR, { recursive: true });
   });