feat: add ownership-safe task deletion

[chenquan]

Summary

• add ownership manifests for Taskflow-created worktrees
• add dry-run/execute/force delete command with lock and preflight safety checks
• update tests, bundled skill, README, and OpenSpec contracts

Validation

• GOCACHE=/private/tmp/taskflow-gocache go test ./...
• GOCACHE=/private/tmp/taskflow-race-cache go test -race ./...
• GOCACHE=/private/tmp/taskflow-gocache go vet ./...
• openspec validate --specs (16/16)
• git diff --check