Prune stale planning artifacts reported by reconcile

[rickardvh]

Dogfooding after the #1218 branch refreshed external intent evidence and reran reconcile. uv run agentic-workspace reconcile --format json reports 12 safe-to-prune planning artifacts:

• 8 closed roadmap candidates for [Workspace]: Extract command-generation as an independent maintainer dependency #1201-[Workspace]: Prepare external command-generation dependency adoption #1208 in .agentic-workspace/planning/state.toml
• 4 closed decomposition records:
  • .agentic-workspace/planning/decompositions/bash-generated-cli.decomposition.json
  • .agentic-workspace/planning/decompositions/github-1124-long-horizon-evaluation.decomposition.json
  • .agentic-workspace/planning/decompositions/github-1130-assurance-requirements.decomposition.json
  • .agentic-workspace/planning/decompositions/powershell-generated-cli.decomposition.json

Expected behavior:

• Apply the command-owned safe prune path rather than hand-editing planning state.
• Verify reconcile no longer reports stale artifacts after pruning.
• Keep this as a separate hygiene PR from feature or bugfix branches.

Suggested command:

uv run agentic-workspace reconcile --apply-safe-prune --format json