Ignore Decodex runtime artifacts during landing

[yvette-carlisle]

Summary

• Treat untracked top-level .decodex-run-activity and .decodex-run-control/ as Decodex runtime artifacts in landing/recovery cleanliness checks.
• Keep ordinary untracked files, nested runtime-looking paths, and tracked runtime marker changes blocking.
• Update runtime spec and review-handoff recovery runbook to match the new clean-source boundary.

Verification

• cargo test -p decodex landing_cleanliness_ignores_untracked_decodex_runtime_markers
• cargo test -p decodex worktree_blocking_status_lines_ignores_untracked_decodex_runtime_artifacts
• cargo test -p decodex merged_worktree_cleanup_debts_treats_decodex_runtime_artifacts_as_clean
• cargo test -p decodex landing_cleanliness_rejects_blocking_worktree_statuses
• cargo make check
• /Users/x/.codex/plugins/cache/hack-ink/playbook/0.3.0/scripts/semantic_drift_audit.py
• git diff --check