Add git indexing: orchestrator, worker, chunking, and embeddings

[karngyan]

Summary

• Implements phase 2 git indexing: orchestrator and worker that clone repos, chunk files, generate OpenAI embeddings, and upsert into Qdrant
• Adds gitrepo library for cloning and traversing Git repositories with provider abstraction (GitHub/GitLab)
• Adds chunking library for splitting Go and generic source files into semantic chunks
• Adds openai library with embeddings client and fake for testing
• Adds gittokens domain for managing per-workspace Git access tokens
• Adds index_runs DB table and queries to track indexing job state
• Expands Qdrant collection helpers and CI workflow

Test Plan

• CI passes (unit + integration tests via make test)
• make dev starts without errors (API + Qdrant + Postgres healthy)
• Indexing worker processes a repo end-to-end and upserts vectors into Qdrant
• GET /health returns healthy