ci: run the existing test suite on push and PR

[hozantaher]

The repo has 92 test files and vitest.config.ts, but .github/workflows/ only contains publish.yml — nothing runs them. Pull requests therefore land with no checks at all (#102 is currently sitting at zero).

This adds a Test workflow mirroring the conventions already in publish.yml (macos-latest, pnpm/action-setup@v4, Node 22.14.0 with pnpm cache, --frozen-lockfile) and running pnpm typecheck then pnpm test. The e2e specs gate themselves behind DMUX_E2E=1 and stay skipped, since they drive a real tmux server.

One test had to be fixed first

main is not currently green. __tests__/integration/gitOperations.test.ts calls:

await generateCommitMessage('diff content here', '/test');

but the function takes a single argument (src/utils/aiMerge.ts:150), which is how production callers use it (mergePopup.tsx:117, commitMessageHandler.ts:25). The first argument was being used as repoPath.

The assertion also never reached the mocked fetch: callOpenRouter returns null when OPENROUTER_API_KEY is unset (aiMerge.ts:36-37), so the flow fell through to callClaudeCode, whose execSync is mocked in that file — the test was asserting against '[main abc123] Test commit'. The fix passes the real signature and sets a dummy key around the call so the OpenRouter branch is actually exercised, restoring the previous value afterwards.

Verified locally: 109 passed | 1 skipped, 652 tests, pnpm typecheck clean.

Unrelated finding

src/components/panes/MergePane.tsx:143 calls generateCommitMessage() with no argument at all, so repoPath is undefined. Left alone here to keep this PR reviewable — happy to file it separately.