feat(checkpoint): add per-agent causal frontiers

[dollspace-gay]

Summary

• replace the global checkpoint watermark with a versioned per-agent causal frontier
• validate contiguous histories, pinned tips, prefix hashes, and checkpoint semantics
• adopt only causally dominant checkpoints and recompute concurrent authority
• automatically migrate legacy checkpoints during daemon reconciliation while preserving eventless records and recovery evidence
• update dashboard readers, reconciliation fixtures, architecture documentation, and bounded cross-platform CI coverage

Verification

• cargo fmt --all -- --check
• cargo clippy --all-targets --all-features -- -D warnings
• cargo clippy --locked -- -D warnings -W clippy::unwrap_used -W clippy::expect_used
• cargo test --all-features --quiet: 5,460 executed tests passed; 4 intentionally ignored
• cargo test --doc --all-features
• cargo build --release --all-features
• cargo check --locked --target x86_64-pc-windows-gnu --all-targets --all-features
• python3 scripts/sync-codex-plugin.py --check
• feature-branch CI run 33823538068 passed

Closes #798