Add 3 MCP tools, h3 splitting, CONTRIBUTING.md, remove TODO.md

[TomMaSS]

Summary

• prd_reorder_sections (prd_reorder_sections tool #33) — reorder sections by slug list, unlisted preserve relative order
• prd_merge_sections (prd_merge_sections tool #32) — merge source into target (content + deps + comments + children), deterministic lock order, cycle detection
• prd_import_url (prd_import_url tool #25) — fetch URL with SSRF protection, Google Docs/GitHub URL rewriting, 1MB cap
• h3 splitting (h3 splitting in markdown import #34) — heading_level >= 3 creates parent sections with parent_section_id linkage and slug dedup
• CONTRIBUTING.md (CONTRIBUTING.md #16) — dev setup, testing, MCP tool pattern, commit guidelines
• AGENTS.md — GitHub Project task workflow with column option IDs
• README.md + docs/tool-reference.md — tool count 31 to 34, new tool entries
• TODO.md removed — fully replaced by GitHub Project Update TODO.md with prioritized improvement roadmap #2 issues

Test plan

• 199 tests passing (28 new: 5 reorder, 10 merge, 8 import_url, 6 h3 import)
• Verify prd_reorder_sections partial reorder preserves unlisted order
• Verify prd_merge_sections transfers deps, comments, children correctly
• Verify prd_import_url SSRF blocks localhost/private IPs
• Verify h3 import creates correct parent_section_id linkage

Closes #16, #25, #32, #33, #34