Add gist ontology layer and improve template documentation

[snapsynapse]

• Add optional gist semantic layer (enabled by default): @type annotations on JSON API items, context.jsonld at api/v1/, and ontology attribution block in index.json; disable with ontology.enabled: false in project.yml
• Fix schema and data bugs: document source_file field, correct source_file paths in mapping/index.yml (container/ -> frameworks/), fix title: -> name: in CONTRIBUTING.md and VERIFICATION.md frontmatter examples
• Clarify secondary entity role in project.yml (inline/mapping-defined, not standalone files)
• Add validate.js error reference table to README
• Add MCP tool naming derivation table to README
• Improve .gitignore comment to explain Option A vs B for forks
• Sharpen Quick Start: fork vs clone distinction, specific first edit guidance, success signal for build step
• Add live demo link and "What it produces" section before examples
• Add ASCII data-flow diagram showing project.yml + data -> build -> docs
• Add "What to keep" callout in Replacing example data section
• Trim README Verification section to pointer; consolidate duplicate config prose in VERIFICATION.md