chore: release v0.19.0

[laynepenney]

Summary

• Remove [[bin]] name = "gr2" from root Cargo.toml (stop shipping gr2 binary with cargo install gitgrip)
• Move gr2 binary entry point into the gr2 subcrate for development/testing
• Bump version to 0.19.0
• Add CHANGELOG entry covering 29 commits since v0.18.0

Premium boundary: core OSS (workspace orchestration tooling).

Test plan

• cargo build passes (gr and gitgrip binaries only)
• cargo build --release passes (no gr2 in release/)
• cargo test passes (all 93+ CLI tests, including gr2 subcrate tests)
• cargo clippy --workspace clean
• Verified target/release/gr2 does not exist

What ships in v0.19.0

gr1 improvements:

• Checkout lifecycle commands (feat: add checkout lifecycle commands #489)
• Cache-backed checkout creation (feat: add cache-backed checkout creation #485)
• Machine-level manifest repo caches (feat: add machine-level manifest repo caches #484)
• gr migrate in-place (feat: gr migrate in-place (#456) #458)
• CI: Windows tests non-blocking (ci: split Windows tests into non-blocking lane #487)

gr2 workspace model:

• Team-workspace spec/plan/apply lifecycle (Sprint 13-18)
• Repo materialization, autostash, partial convergence (feat: extend gr2 apply beyond initial ExecutionPlan slice #514, feat: gr2 apply preserve local modifications with autostash policy #534, feat: gr2 apply converges partially-materialized units with missing nested repo checkouts #539)
• Design docs and status surfaces (docs: design gr2 repo maintenance and lanes #543, feat: add gr2 repo status surface #548, feat: add gr2 lane metadata surface #549, docs: ratify gr2 UX model and workflow #557, feat: merge atlas gr2 design stack (exec surface + prototypes) #558, proto: add identity-unit binding seam #560)

Binary change:

• gr2 binary removed from main crate release