v0.4.0: 4 new providers, 18 stability fixes, 360 tests

New providers: OpenCode, Qwen Code, Ollama, LocalAI (12 total)

Brainstorm stability (B1-B9): silence-based timeout, auth pre-check,
synthesis fallback, oscillation detection, convergence regex broadening,
empty contribution guard, stopword-based similarity, duplicate agent
validation, cancel propagation.

@-mention stability (M1-M8): question timeout, max mentions limit,
refined parsing regex, file mention boundary matching, agent validation,
workspace file cache refresh, file resolution warnings, retry cleanup.

Bug fixes:
- Windows spawn EINVAL — auto shell mode (closes #14)
- Codex codexPath ignored in brainstorm (closes #26)
- Full-path file mention matching

Test infrastructure: 360 automated vitest tests with mock providers.

Closes #14, #24, #25, #26

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Merge PR #15: feat: Add custom model configuration and provider settings

Incorporates Patrick Fu's contribution for custom model configuration
and provider-specific settings. This functionality has been expanded
upon and is included in the current codebase.

Co-Authored-By: Patrick Fu <me@paaatrick.com>