feat: remove open command

[chenquan]

Summary

• Remove the taskflow open command and its Go launch path, including Service.Open, preflightOpen, internal/devtool, and PATH lookup.
• Rework command and E2E tests to assert create --dry-run reuse for dirty matching worktrees.
• Generate native claude/codex commands from the bundled skill and update the README and OpenSpec task state.
• Make native launch guidance shell-safe for POSIX shells, PowerShell, and cmd.exe, including path quoting and escaping.
• Require a post-create dry-run without --repo to confirm every worktree is reuse before composing a native command.
• Add the readiness-and-initialization-integrity spec delta so the removed Open source-identity requirement is fully represented.

Validation

• openspec validate --all --strict --no-interactive
• go build ./...
• go vet ./...
• go test ./...
• go test -race ./...
• git diff --check