add zenodo DOI

GCC v2: lean git-backed memory with utility scripts

Replace verbose markdown storage (~500 tokens/entry) with lean
index.yaml format (~50 tokens/entry) backed by real git commits.
Context is reconstructed on demand via git show.

New scripts:
- gcc_commit.sh: real git commit + lean index entry
- gcc_context.sh: reconstruct context from hashes
- gcc_bridge.sh: feed commit data to aiyoucli vector memory
- gcc_cleanup.sh: TTL-based worktree cleanup + index pruning

Updated gcc_init.sh with dual mode (git/standalone) detection.
Updated SKILL.md, README.md, file_formats.md, sample_session.md
for v2 format and workflows.