release: v0.21.4

[bug-ops]

Summary

• Bump version from 0.21.3 to 0.21.4
• Update CHANGELOG.md with release section dated 2026-06-05
• Update test badge count: 10294 → 10511
• Update splash screen snapshot for new version string
• Refresh book documentation (cli reference, configuration reference, wizard guide)
• Update specs: LLM providers, skills subsystem, worktree subsystem

Checklist

• Version updated in all manifests ([workspace.package] + all [workspace.dependencies] entries)
• Cargo.lock regenerated
• CHANGELOG.md has [0.21.4] section with date
• Test badge reflects current count (10511)
• Insta snapshot updated for version string change
• All CI checks pass locally (fmt, clippy, nextest, RUSTFLAGS=-D warnings)
• Rebased on main (includes refactor(agent-tools,db): replace send_stop_hint with StopHint enum; fix sql! import guards #4863)
• Ready for tagging after merge