feat: simplify Taskflow to worktree create and CLI open

[chenquan]

Summary

• simplify the public runtime to create and open (plus version)
• replace persisted state/report lifecycle with live Git worktree reconciliation
• preserve multi-repository creation, safe locking, dry-run/execute, and Codex/Claude launch flows
• update the skill, README, specs, tests, and archive the completed OpenSpec change

Breaking changes

• remove init, start, status, validate, repo add, and the skill command
• remove state.json, validation reports, and repository inventory as runtime dependencies
• existing task workspaces using the old lifecycle must be recreated under the new contract
• add repositories with create --repo when creating a task

Validation

• openspec validate --all --strict --json
• go test ./...
• go vet ./...
• go test -race ./...
• go test ./cmd -run 'TestE2E' -count=1
• go build ./...
• compiled-binary smoke test for create --execute and open --tool codex