feat(aidd-pm): spike skill — a timeboxed question with a written answer

[blafourcade]

Add a spike skill to aidd-pm. A spike is an artifact: a bounded research question, a timebox, and a written answer that unblocks estimation or a design decision. Same criterion that earns story-map its own skill.

Extracted from #147 (grooming), where it was one action among six. Grooming operates on a backlog; a spike produces a document.

Scope

• Frame the unknown as one answerable question, with the decision it unblocks.
• Set a timebox and the exit condition: what result ends the spike, including a negative one.
• Write the outcome: what was tried, what was learned, what the team should now do.

Acceptance criteria

• Takes an issue or a plain question as input; refuses a question with no decision attached to it.
• Persists the spike as its own document, not as a comment inside another artifact.
• Names the follow-up: the estimate it unblocks, or the design it settles.
• A spike that ran out of timebox without an answer still produces a written outcome.

Origin

Surfaced by #355 (closed), which bundled acceptance criteria and spikes into one skill. Acceptance criteria already ship in aidd-pm:03-prd and aidd-pm:04-spec; the spike had no home.