test(acp): cover OpenCode additional directory fallback

[Yevanchen]

What changed

• add a no-key contract test against the pinned OpenCode 1.18.4 ACP binary
• assert OpenCode omits sessionCapabilities.additionalDirectories
• verify a non-empty directory request degrades by omitting the field and still creates session/new
• isolate HOME/XDG state and avoid inheriting provider credentials

Why

PR #86 fixed the ACP outage with a mocked regression test. This adds executable evidence against the real OpenCode initialize/session contract so future capability drift is caught before model requests.

Validation

• bun test tests/opencode-acp-contract.test.ts tests/acp-session-setup.test.ts — 8 passed
• bun run tc — passed
• vp lint tests/opencode-acp-contract.test.ts — passed
• bun run build — passed
• bun run check — formatting, lint, typecheck, and the new test passed; the macOS run still has 12 unchanged process-watchdog/path failures outside this diff (1201 passed, 61 skipped)

Generated files

None. No dependency, lockfile, GraphQL, or database changes.