[Feature] Refine 99problems skill for progressive disclosure, optional subagent research, and org conventions template

[mbe24]

Problem

The current 99problems skill can be improved in three areas:

1. progressive disclosure can be tighter (keep SKILL.md lean and shift details to references),
2. research positioning is implicit (issue/PR retrieval should be clearly framed as a viable research avenue for topic discovery),
3. organization-specific query conventions need a cleaner opt-in mechanism.

Design

Refine the template skill to:

1. keep SKILL.md concise and focused on core workflow,
2. explicitly state that issue/PR context retrieval is valid for research tasks (in addition to source-file inspection and web search),
3. add an optional subagent-based research workflow for context-budget efficiency,
4. add references/CONVENTIONS_TEMPLATE.md and define opt-in activation by copying it to references/CONVENTIONS.md.

Scope

• Update templates/skills/99problems/SKILL.md.
• Add templates/skills/99problems/references/CONVENTIONS_TEMPLATE.md.
• Add clear progressive-disclosure load guidance for REFERENCE.md, FORMS.md, and optional CONVENTIONS.md.
• Document evidence expectations for subagent summaries (IDs/keys and commands/queries used).

Boundary

• No runtime CLI/provider behavior changes.
• No direct edits to generated .agents/skills/99problems in this issue.
• No automatic file-copy mechanism for conventions activation in this phase.

Acceptance Criteria

• SKILL.md remains concise and under 500 lines.
• SKILL.md clearly positions issue/PR history as a research avenue.
• Optional subagent workflow is documented with evidence-based summary requirements.
• CONVENTIONS_TEMPLATE.md exists and is explicitly marked as inactive until copied to CONVENTIONS.md.
• Conventions template includes org query/label/status fields and a "no secrets" note.

Context

This improves skill quality and context efficiency while staying aligned with Agent Skills progressive-disclosure guidance.