docs: add "Progressive Disclosure Skills" guide

Context

The Neo Agent OS uses a Progressive Disclosure pattern for agent skills — on-demand loaded instruction sets that minimize system prompt token bloat while giving agents specialized procedural knowledge. Seven skills exist today, but there is no documentation explaining:

• What the skill pattern is and why it exists
• How the three lifecycle skills compose into a workflow
• How to create new skills
• The token economics behind lazy loading vs. system prompt injection

Proposed Guide: learn/agentos/ProgressiveDisclosureSkills.md

Content Scope

1. The Progressive Disclosure Pattern — YAML frontmatter contract, folder structure (SKILL.md, references/, assets/), trigger-based activation
2. Token Economics — Why skills exist: system prompt budget. Loading a 4K-token workflow guide on-demand vs. bloating every session with all 7 skill payloads
3. The Lifecycle Triad — How ticket-intake → pull-request → pr-review compose:
   • ticket-intake: Pre-Execution Reflection Gate (validation sweep, ROI calculation, rejection protocol)
   • pull-request: Post-Implementation Gate ("Stepping Back" reflection, branch mandate, Conventional Commits, state handoff)
   • pr-review: Quality Gate (evaluation metrics, graph ingestion tags, LGTM/Required Actions)
4. Tactical Skills — neural-link (live inspection), unit-test (Playwright integration)
5. The Meta-Skill — create-skill bootstraps new skills with correct structure
6. How Skills Compose with AGENTS.md — Skills are invoked by mandate rules in the root system prompt. The system prompt says when to invoke; the skill says how to execute.
7. Adding New Skills — Step-by-step via the create-skill skill

Skill Inventory

Registration

• Add to learn/tree.json under Agent OS
• Cross-link from Swarm Intelligence (execution phase)
• Cross-link from Strategic Workflows (skill invocation patterns)

Acceptance Criteria

• Guide created at learn/agentos/ProgressiveDisclosureSkills.md
• Registered in tree.json
• Cross-linked from Swarm Intelligence and Strategic Workflows
• CodebaseOverview cross-reference updated

A2A Context

• Parent: #9981 (Architecture formalization epic)
• Sibling: #9983 (Swarm Intelligence — completed, PR #9984)
• Sibling: Dream Pipeline ticket (created in same session)
• Origin Session: 70334eab-72c9-44a6-8f48-0b6a96604f49