[AIAA] Build step 9: command workflow

[nate-double-u]

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.