test: migrate from AVA to Node.js native test runner (#346)

AVA stores snapshots in a binary format (`.snap`), which produces no
meaningful diffs and bloats Git history. This replaces AVA with the
built-in `node:test` module, whose snapshot support generates
human-readable text files that are easy to diff and review in pull
requests.

The migration also replaces `@sinonjs/fake-timers` and `execa` with
Node.js built-ins (`node:test` mock timers and `node:child_process`),
removing three dev dependencies total.

### Changes

- **`tests/index.js`**: Rewritten to use `node:test` with a custom
snapshot serializer that renders strings with actual newlines. Uses
subtests for labeled `stderr`/`stdout` snapshots, and only snapshots
non-empty output.
- **`tests/main-repo-skew.test.js`**: Replace `@sinonjs/fake-timers`
with `mock.timers.enable()` from `node:test`.
- **`tests/README.md`**: Updated documentation to reflect `node --test`
and the new snapshot file.
- **`package.json`**: Remove `ava`, `@sinonjs/fake-timers`, and `execa`
from devDependencies. Update test script to `c8 --100 node --test
tests/index.js`.
- **`tests/index.js.snapshot`**: New text-based snapshot file replacing
binary `tests/snapshots/index.js.snap`.
- **`tests/snapshots/`**: Deleted.

All 22 test scenarios (66 subtests) pass with 100% code coverage.

Closes #344

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

package.json

@@ -10,7 +10,7 @@
   "packageManager": "npm@10.9.4",
   "scripts": {
     "build": "esbuild main.js post.js --bundle --outdir=dist --out-extension:.js=.cjs --platform=node --packages=bundle",
-    "test": "c8 --100 ava tests/index.js",
+    "test": "c8 --100 node --test tests/index.js",
     "coverage": "c8 report --reporter html",
     "postcoverage": "open-cli coverage/index.html"
   },
@@ -24,12 +24,9 @@
   },
   "devDependencies": {
     "@octokit/openapi": "^21.0.0",
-    "@sinonjs/fake-timers": "^15.1.0",
-    "ava": "^6.4.1",
     "c8": "^10.1.3",
     "dotenv": "^17.3.1",
     "esbuild": "^0.27.3",
-    "execa": "^9.6.1",
     "open-cli": "^8.0.0",
     "yaml": "^2.8.2"
   },