Implement Cargo feature flags to make the CLI binary modular:
- default = [] (minimal: TUI + exec + ACP only)
- full = all features for complete functionality
- Individual features: app-server, cloud-tasks, login, mcp-server, chatgpt, responses-api-proxy

This reduces release binary size from 46MB to 36MB (22% reduction) for
minimal builds by excluding optional HTTP-based functionality.

Key changes:
- Made 6 dependencies optional with dep: prefix
- Added #[cfg(feature = "...")] guards to main.rs imports, enum variants, and match arms
- Added required-features to MCP integration tests
- Updated docs.md with feature flag documentation
🤖 Generated with [Nori](https://nori.ai)

Co-Authored-By: Nori <contact@tilework.tech>

Add conditional compilation throughout TUI crate to enable modular
builds:

- Define feature flags in Cargo.toml: full, login, feedback, backend-client, upstream-updates
- Make codex-login, codex-feedback, codex-backend-client optional dependencies
- Gate onboarding auth flow with #[cfg(feature = "login")]
- Gate feedback popups and events with #[cfg(feature = "feedback")]
- Gate BackendClient usage with #[cfg(feature = "backend-client")]
- Hide /logout and /feedback slash commands when features disabled
- Add nori/feedback.rs to redirect feedback to GitHub Discussions
- Add nori/update_action.rs and nori/updates.rs for Nori update system
- Gate feedback-related tests with #[cfg(feature = "feedback")]
- Create FEATURE_PLAN.md documenting the implementation plan

This enables the Nori CLI fork to default to ACP-only mode without
OpenAI-specific functionality, while allowing full-featured builds
via --features full.

Add missing re-export of nori::updates as crate::updates when
upstream-updates feature is disabled. This fixes a release build error
where app.rs calls crate::updates::get_upgrade_version() but the module
wasn't available in the Nori configuration.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

- Add `oss-providers` feature flag to gate codex-ollama and codex-lmstudio
  - CLI, TUI, and common crates now support `oss-providers` feature
  - OSS provider functions return stubs when feature disabled
- Improve feedback_compat.rs stub for API compatibility
  - Match anyhow::Result<()> return type for upload_feedback
  - Add proper tests with feature-gated assertions
  - Add placeholder comments for future Nori feedback integration
- Include `oss-providers` in `full` feature bundle
- Update documentation in docs.md files

This continues the feature flags work to enable minimal binary builds
by making OSS provider dependencies optional.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>