feat(aidd-pm): discovery skill — brief, North Star, personas, JTBD

[blafourcade]

Add the upstream discovery skill aidd-pm is missing: everything a PM produces before a PRD exists. Absorbs #350 (personas + JTBD), which described the same conversation.

Why one skill, not four

Per R2 of the skill-authoring contract, one skill = one domain. Product brief, North Star, personas and Jobs-To-Be-Done are the four artifacts of a single activity: framing who the product serves and what success means, before requirements exist. They are produced in one sitting, from one conversation.

Scope

• Product brief: the problem, the audience, the bet.
• North Star: the one metric that says the bet paid off.
• Personas: who the users are.
• Jobs-To-Be-Done: what they hire the product to do.

Acceptance criteria

• One skill, four artifacts, saved under the project's docs directory.
• Output feeds aidd-pm:03-prd without reformatting.
• The skill runs from a blank slate (greenfield) and from an existing product (the artifacts are inferred, then confirmed).

Placement

aidd-pm, before 03-prd in the flow. Naming follows the artifact domain, not the method.