docs: accept Scient documentation system

[yaacovcorcos]

Summary

• records the accepted cross-repository documentation authority and publishing contract
• preserves the complete Markdown, maintainability, and capability-to-foundation audits
• defines staged implementation, pilot gates, anti-complexity constraints, and explicit exclusions

Boundaries

• no corpus migration or website publication
• no Scient-agent inspection or change
• no MCP or product-architecture implementation

Validation

• Markdown formatting check for all six changed files
• local Markdown link check for all six changed files
• git diff --check

[yaacovcorcos]

Hosted Actions could not start because of the account billing/spending-limit error. Per docs/operations/github-operating-model.md, I ran the exact evidence-manifest workflow locally on the PR head with Node v24.19.0: bootstrap syntax/idempotency passed; Gate 1.5 manifest passed; all 20 owned-source evidence tests passed; owned-source consistency passed; git diff --check passed; worktree remained clean.