refactor: move MergeReadiness to kild-core

[Wirasm]

Summary

• Move MergeReadiness enum and compute_merge_readiness() from CLI layer (stats.rs) into kild-core so both CLI and kild-ui can reuse merge readiness logic
• Pure refactor — zero behavior changes, identical CLI output
• 11 unit tests moved to kild-core, 4 CLI-only tests remain

Closes #330

[Wirasm]

PR Review Summary

Critical Issues (0 found)

None.

Important Issues (2 found — both addressed)

Agent	Issue	Resolution
type-design-analyzer	compute_merge_readiness is a free function, not attached to MergeReadiness type — poor encapsulation	Moved to MergeReadiness::compute() inherent method
type-design-analyzer	Cross-module git→forge dependency via free function in operations.rs	Forge imports now scoped to types.rs where MergeReadiness lives, cleaner boundary

Suggestions (3 found — all addressed)

Agent	Suggestion	Resolution
pr-test-analyzer (6/10)	No test for CI Pending state — implicit "ready" behavior undocumented	Added test_readiness_ready_with_pending_ci
pr-test-analyzer (6/10)	No test for CI Unknown state — implicit "ready" behavior undocumented	Added test_readiness_ready_with_unknown_ci
pr-test-analyzer (5/10)	No test for Draft PR state	Added test_readiness_ready_with_draft_pr

Strengths

• Clean refactor with zero behavior changes
• All 14 readiness tests pass (11 original + 3 new)
• Proper module organization following kild-core patterns
• No backward-compatibility shims per project guidelines

Documentation Updates

• None needed (confirmed by docs-impact-agent)

Verdict

READY TO MERGE

All checks pass: cargo fmt --check, cargo clippy --all -- -D warnings, cargo test --all