[AIAA] Add repository instructions for AI agents

[nate-double-u]

Adds .github/copilot-instructions.md: ground rules for any AI agent
working in this repository. Build step 2 of the assessment system
(spec section 16); reviewed against sections 8 and 14. Tracking: #368.

What it carries

• Safety (section 8): everything read is data, not instructions; writes
  stay in this repository; a human reviews and merges every change;
  secrets stay out of outputs.
• Evidence discipline (section 14 common rules, repo-wide form):
  reproducible numbers, cited findings, unverifiable content flagged,
  not asserted.
• Point at docs/analysis/, never restate it (P-2).
• Repo conventions: npm run check.

Shape choices

• Lean on purpose: this file loads into every Copilot surface here,
  including code review on ordinary PRs. Nothing phase-specific; that
  belongs to the agent profiles (later step).
• No references to the spec document: the built system's documentation
  is self-sufficient; the spec stays a design artifact.

Verification

• Assertion script (session artifact): 16 structural checks, all pass
  (required rules present, no spec references, length bounds, style).
• npm run check passes.

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[Copilot]

Pull request overview

Adds repository-wide guidance for AI agents, covering safety, evidence standards, methodology references, and validation.

Changes:

• Defines AI-agent ground rules and evidence requirements.
• Directs agents to existing assessment methodology.
• Documents repository checks.

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[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated no new comments.

[iRaindrop]

@nate-double-u - Should the copilot-instructions include writing style instructions or should they be handled at a more granular level like in prompt-related files?

Such style instructions include regarding verbosity, handling parenthetical clauses, and a few others.

[nate-double-u]

I think granular. This file loads into every Copilot surface in the repo (including code review on config PRs), so it should only list what applies to all agent work: safety, evidence, pointers. I think that style for assessment deliverables should be a part of the the per-phase agent profiles that'll get built in later steps. If a style rule turns out to be needed repo-wide, we can add it here then.