feat: add /document-generate skill + enhance /document-release with Diataxis coverage map

[garrytan-agents]

What

Inspired by @doodlestein's documentation-website skill and the FrankenTUI docs site it produced. Two changes:

1. New /document-generate skill — writes missing documentation from scratch using the Diataxis framework
2. Enhanced /document-release — now audits documentation coverage gaps and suggests /document-generate as follow-up

These two skills compose like Unix pipes: /document-release finds what's missing, /document-generate fills the gaps.

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New: /document-generate

Writes high-quality documentation from scratch for a feature, module, or entire project. Uses the Diataxis framework (tutorial / how-to / reference / explanation).

9-step workflow:
0. Scope and Intent — confirm target, output format, where docs go

1. Codebase Archaeology — deep research phase, reads code + tests + comments before writing anything
2. Diataxis Partitioning — decide which quadrants each entity needs (decision matrix included)
3. Write Reference — factual, complete, code-derived (foundation layer, written first)
4. Write Explanation — design rationale, trade-offs, "why" docs with ASCII diagrams
5. Write How-Tos — task-oriented guides with prerequisites, steps, verification, troubleshooting
6. Write Tutorials — learning-oriented, hard rule: working result within 3 steps
7. Cross-Document Linking — ensure discoverability from README within 2 clicks
8. Quality Self-Review — accuracy, completeness, and voice gates
9. Commit and Output — structured summary with quadrant breakdown

Key design principles stolen from @doodlestein:

• Research-first, write-second (the "Partition" phase maps everything before drafting)
• Reference docs written first (establish vocabulary for other quadrants)
• Composable with other skills (can be called standalone or from /document-release)
• Ambitious scope as a feature ("boil the lake" — write complete docs, not minimal)

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Enhanced: /document-release

Step 1.5: Coverage Map (Blast-Radius Analysis)

New step between Pre-flight and Per-File Audit. Scans the diff for new public surface and assesses documentation coverage across Diataxis quadrants. Flags gaps but never auto-generates — suggests /document-generate when significant gaps are found.

Architecture Diagram Drift Detection

Parses entity names from ASCII/Mermaid diagrams, cross-references against the diff. Flags stale diagrams.

Enhanced CHANGELOG Sell Test

0-3 Diataxis rubric: +1 for "what changed" (reference), +1 for "why care" (explanation), +1 for "how to use it" (how-to). Entries scoring less than 2 get rewritten.

Documentation Debt in PR Body

Step 9 now appends a ### Documentation Debt section with critical gaps, common gaps, and stale diagrams.

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Files Changed

• document-generate/SKILL.md.tmpl — NEW template for /document-generate
• document-generate/SKILL.md — generated output
• document-release/SKILL.md.tmpl — enhanced template
• document-release/SKILL.md — regenerated output