feat: Add lazy plugin loading and version validation

[finxo]

Pull Request

📝 Summary

Defers plugin initialization until first use, preventing startup and screen refreshes from paying for plugin I/O that may never be needed. Adds plugin/core version compatibility validation and makes official plugin versions explicit and test-enforced.
Breaking changes: None.

🔧 Changes Made

• Split metadata discovery, loading, and thread-safe first-use initialization; disabled plugins remain unloaded unless explicitly viewed.
• Update TUI, workflow, and config call sites to initialize only required plugins; plugin management still loads all plugins for accurate status.
• Add titan_requires compatibility checks, package-level plugin versions, and repository tests to prevent version-contract drift.
• Replace GitHub’s networked gh auth status initialization check with local gh auth token.

🧪 Testing

• Unit tests added/updated (poetry run pytest)
• All tests passing (make test)
• Manual testing with titan-dev

Added coverage for lazy initialization, dependency ordering, failure retries, thread safety, metadata-only discovery, on-demand disabled-plugin loading, import-failure logging, and official plugin version/requirement synchronization.

📊 Logs

• No new log events

• plugin_registry_not_prepared (DEBUG) — plugin name when initialization is requested before registry preparation.
• plugin_incompatible (WARNING) — plugin name and compatibility reason when its Titan CLI requirement is unmet.

✅ Checklist

• Self-review done
• Follows the project's logging rules (no secrets, no content in logs)
• New and existing tests pass
• Documentation updated if needed
• Plugin documentation updated when plugin functions or parameters changed (Plugins > Git Plugin, GitHub Plugin, Jira Plugin)