refactor: consolidate discrete skills into single convex skill with references

[bencered]

Summary

Consolidates 14 separate skill directories into a single skills/convex/ skill with a references/ folder, following the pattern from cloudflare/skills. This is a v2.0.0 breaking change.

Before

skills/convex-functions/SKILL.md
skills/convex-agents/SKILL.md
skills/convex-best-practices/SKILL.md
... (14 directories, each with duplicate assets + openai.yaml)

After

skills/convex/
├── SKILL.md              ← decision trees + reference index
├── references/           ← 13 focused markdown files
├── assets/               ← shared logo files
└── agents/openai.yaml    ← single Codex config

Why

• Progressive disclosure — agents load the main SKILL.md first (~100 tokens of metadata at startup), then drill into specific references only when needed. Matches the agentskills.io spec design philosophy.
• Ecosystem compat — tested with npx skills add (discovers 1 skill, installs to 42 agents), npm pack, Claude Code plugin, and Cursor plugin.
• Less duplication — removed 13 copies of logo assets and openai.yaml.

Breaking Changes

• npm API: SKILLS keys changed ("convex-functions" → "functions"). A backward-compat alias with both old and new keys is kept, but prefer REFERENCES. getSkill("convex-best-practices") now throws with a helpful migration message.
• CLI: convex-skills install <skill-name> replaced by convex-skills install (warns if old name passed). install-all kept as alias.
• CLAUDE.md removed from npm package — was a symlink to agents.md, which is now included directly.

See the changelog for full migration guide with before/after code examples.

What Changed

• 13 SKILL.md files → skills/convex/references/*.md (frontmatter stripped)
• New SKILL.md with decision trees and reference index
• index.js / bin/cli.js rewritten for consolidated structure
• Added .claude-plugin/marketplace.json, .cursor-plugin/
• SKILL.md frontmatter now spec-compliant (compatibility, allowed-tools, metadata)
• getReference(), listReferences(), getSkillPath() resolve old names gracefully
• Cleaned up broken .codex/skills/ symlinks, .DS_Store, stale docs
• Fixed CONTRIBUTING.md license (MIT → Apache-2.0), CLAUDE.md/AGENTS.md symlinks
• Updated docs.md, files.md, changelog.md, README.md, agents.md, .codex/README.md

Testing

node bin/cli.js list                          # ✅ 1 skill, 13 references
node bin/cli.js install --dir /tmp/test       # ✅ copies SKILL.md + 13 refs
node bin/cli.js install convex-functions      # ✅ warns + installs full skill
npx skills add . --all -y                     # ✅ 1 skill → 42 agents
npm pack --dry-run                            # ✅ 29 files, 62.6 kB