[good first issue] Add a schema reference page to docs/ #2

Summary

Create a single-page reference (docs/schema-reference.md) that lists every field in tweet.json v1, the agent bundle v1.0, and the manifest v1.0, with type, required/optional status, and a one-line description.

Context

Currently, field definitions are spread across three docs:

• docs/agent-bundle-spec.md (bundle fields)
• docs/provenance.md (manifest fields)
• Inline in tools/scripts/tweet_schema.py (tweet.json fields)

A consolidated reference page helps both human contributors and AI agents quickly look up any field without reading multiple documents.

What needs to be done

• Read tools/scripts/tweet_schema.py to extract all tweet.json fields
• Read docs/agent-bundle-spec.md section 3 for bundle fields
• Read docs/provenance.md section 2 for manifest fields
• Create docs/schema-reference.md with three tables (one per schema), columns: Field | Type | Required | Description
• Add a link to the new page in README.md documentation table
• Verify all field names match the actual JSON Schema files in schemas/

Skills helpful

• Markdown
• JSON (reading schema files)

No coding experience required -- this is a documentation task.

How to verify

# Ensure the doc renders and links are valid
python -c "from pathlib import Path; p=Path('docs/schema-reference.md'); print('exists:', p.exists(), 'size:', p.stat().st_size)"

# Verify schema field names match
cd tools && python citeseal.py lint && cd ..

Checklist

• This task does not involve bulk-scraping or bypassing platform access controls.
• I have checked existing issues for duplicates.