Plans and specs have no completion status after execution

After an agent executes a plan (via executing-plans or subagent-driven-development), the .md plan and spec files are left unchanged — all checkboxes remain - [ ], with no indication that the work was completed.

This becomes a problem when a user returns after a few days and wants to know: Is there anything left to do? Currently, the only way to verify is to cross-reference git log against each plan file manually.

Suggested fix: After development is complete, have the agent (or a finishing subagent) update the plan/spec files to mark them as done. Some options:

• Add a > ✅ Completed on YYYY-MM-DD banner at the top of the file
• Check off the - [ ] checkboxes to - [x]
• Or update a status field in a central index file

Any of these would let a returning user scan the docs/ folder and immediately know whether there's unfinished work.