docs: surface README TOC, clarify receipt agent optionality, add v1.0.0 changelog, split CONTRIBUTING, remove redundant ajv entrypoint

[GsCommand]

Motivation

• Improve first impressions and navigation by moving the README Table of Contents to the top-level intro area so readers can jump to sections immediately.
• Fix a contract-document mismatch by making the README receipt description precise and explicitly marking agent as optional so docs match the schema contract.
• Provide a complete historical record by adding a v1.0.0 entry to CHANGELOG.md.
• Reduce tooling ambiguity and surface a single canonical validation workflow while separating contributor onboarding from contribution rules.

Description

• Moved the README Table of Contents immediately after the title/purpose block and adjusted surrounding README navigation and validation guidance to recommend npm run validate as the canonical command.
• Tightened the README receipt field language so the required spine no longer implies agent is mandatory and updated the receipt examples to show agent as optional while calling out result_hash/result_cid as optional evidence fields.
• Appended a ## 1.0.0 initial release entry to CHANGELOG.md describing the original release and its relationship to the current v1.1.0 line.
• Split contribution content by adding a new CONTRIBUTING.md with a strict PR spine, commit conventions, required checks, schema-change requirements, and versioning expectations, and updated ONBOARDING.md to be focused on orientation and working norms.
• Removed the redundant validate:schema package script and deleted scripts/ajv-run.mjs, leaving validate, validate:schemas, and validate:examples as the clear validation entrypoints and updated package.json and README to match.

Testing

• Ran npm run validate which executes npm run validate:schemas and npm run validate:examples, and observed success with ✅ All 45 schemas compiled successfully. and ✅ All example validations completed..
• The targeted commands npm run validate:schemas and npm run validate:examples were exercised as part of the validate run and completed successfully.

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Codex Task