feat(files): search the Files panel by name or path

GET /api/sessions/:id/files gains an optional `q`. With one, the endpoint
answers a FLAT match list instead of a nested tree; without one, the response is
exactly what it was, so every existing caller is untouched.

compileFileQuery() (src/utils/file-query.ts) turns the query string into a
reusable predicate, so the walk prunes as it goes rather than streaming the
whole tree to the client to be filtered there. An empty or whitespace-only
query compiles to null, which is what makes "no query" and "blank query" the
same thing.

The search walk deliberately recurses past directories that do not match — a
file whose ancestors don't match is exactly what people are searching for — so
it carries its own maxMatches cap on top of the existing maxFiles and maxDepth
ones, and reports `truncated` when it stops early. Hidden-file and
excluded-directory rules are the same ones tree mode already applies.

Tests: file-query.test.ts covers the matcher; routes/file-search-mode.test.ts
drives the endpoint against a real temp tree and pins the two properties worth
having — that the walk reaches a match under non-matching parents, and that an
absent or whitespace query leaves the tree response alone. Gating the recursion
on a match turns those red.

src/utils/index.ts

@@ -35,3 +35,5 @@ export { resolveCodexDir, isCodexAvailable } from './codex-cli-resolver.js';
 export { resolveGeminiDir, isGeminiAvailable } from './gemini-cli-resolver.js';
 export { resolveAntigravityDir, isAntigravityAvailable } from './antigravity-cli-resolver.js';
 export { resolvePiDir, isPiAvailable, getPiCliVersion } from './pi-cli-resolver.js';
+export { compileFileQuery, matchFileQuery } from './file-query.js';
+export type { FileQueryMatcher } from './file-query.js';