fix(deepagents): grep/glob match when path points directly at a file (#713)

## Summary
- `grep`/`glob` on `StateBackend`/`StoreBackend` returned zero matches
whenever `path` pointed at an exact file (no glob) — the shared
`validatePath` helper forced a trailing `/` before prefix-matching, so a
file's own key could never match itself+`/`.
- Added a `filterFilesByPath` helper that checks for an exact key match
in the files map first, falling back to the existing directory-prefix
behavior otherwise.

## Test plan
- [x] Added regression tests in `utils.test.ts`: exact-file path (no
glob) for grep and glob, plus directory + glob/filename non-regression
cases for both.
- [x] `pnpm --filter deepagents test` — all tests pass.