[AIAA] Build step 9: command workflow

Build step 9 (spec section 16, PR #363):
`.github/workflows/assessment-commands.yml` plus the comment parser in
`scripts/assessment/`, the `issue_comment` glue behind the slash
commands (section 12).

Done when parser unit tests cover recognition, argument handling, and
every section 12 action and authorization decision, including:

- [ ] An unauthorized commenter refused with a visible reply.
- [ ] A second `/accept` for a project whose assessment is in flight
refused with a pointer to the open intake.
- [ ] `/ready` refused without the `verified` label.
- [ ] `/confirm` applying `confirmed` from the acceptance-frozen set,
refused from a name outside it.
- [ ] A manually applied label honored.

Live wiring is verified on each command's first real use, with manual
bookkeeping as the fallback (HC-4); recorded here.