ci: split optional dependency profiles

[Shadowell]

Summary

• split runtime dependencies into base, Kronos, MCP, training, and CI profiles
• keep Torch sequence training lazy so the core API starts without model packages
• add pip caching, concurrency cancellation, least-privilege permissions, and scoped triggers
• add a dedicated model workflow for Kronos imports and Torch sequence tests
• document installation profiles in README

Verification

• clean requirements-ci.txt virtual environment: 133 passed, 2 skipped
• dependency profile contract tests: 4 passed
• both workflow YAML files parse successfully
• pip check passed in the clean CI environment
• service restarted; root endpoint returned HTTP 200

Closes #5