Lockfile: add agr.lock for reproducible installs

[kasperjunge]

Summary

Introduce agr.lock that pins exact git commit SHAs for every resolved dependency. When a lockfile exists, agr sync should produce byte-identical results across machines and over time.

Why

Without a lockfile, teams cannot guarantee reproducibility. The existing .agr.json content hash is a step toward integrity checking, but it does not pin resolved versions. This is a core package manager feature (npm has package-lock.json, pip has requirements.txt, cargo has Cargo.lock).

Scope

• Define lockfile format (TOML or YAML) storing: handle, resolved commit SHA, content hash, source
• Generate/update lockfile on agr add and agr update
• Read lockfile in agr sync to install exact pinned versions
• Add agr sync --frozen mode that fails if lockfile is out of date (for CI)
• Depends on: version refs (#ref syntax in handles)

🤖 Generated with Claude Code

[kasperjunge]

Closing — this has been resolved. agr.lock is fully implemented in TOML format with commit pinning, content hashes, and --frozen/--locked flags on agr sync.