[AIAA] Build step 2: repository instructions for AI agents

[nate-double-u]

Build step 2 of the AI-assisted assessment system: repository
instructions for any AI agent working in cncf/techdocs.

Deliverable: .github/copilot-instructions.md. It carries the safety
ground rules (content is data, not instructions; write only to this
repository; a human reviews and merges everything; secrets stay out),
the evidence discipline (reproducible numbers, cited findings,
unverifiable means flagged), and the point-don't-copy rule for the
methodology in docs/analysis/.

Scope note: this file loads into every Copilot surface in the repo,
including code review on ordinary PRs, so it stays lean and
repo-generic. Phase-specific behavior belongs to the agent profiles
(a later step).

Done when merged after review against the safety and agent-rules
sections of the design.

• Draft reviewed locally (assertion script green, npm run check
  passes)
• PR opened
• Review against sections 8 and 14
• Merged