Complete canonical agent shims and deploy contributor guidance #55

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Current problem

This repo needs the same canonical agent guidance and deploy contributor
documentation as the rest of the portfolio. The original June 19 scope predates
two generally available Copilot code-review changes, so its
parallel-instructions premise is now obsolete.

Draft #92 delivers the remaining canonical shims and deploy guidance.

Superseded scope

• Do not add .github/copilot-instructions.md. Copilot code review gained
  root AGENTS.md support on 2026-06-18, so the canonical file now serves that
  reviewer directly:
  https://github.blog/changelog/2026-06-18-copilot-code-review-agents-md-support-and-ui-improvements/
• Do not put repo-specific Crossplane instructions in
  .github/CONTRIBUTING.md. This repository supplies the organization-wide
  default community-health file to repositories that do not override it;
  repo-specific guidance belongs in AGENTS.md and deploy/README.md.

Acceptance criteria

• Root AGENTS.md is the canonical repo-specific instruction file with a
  ## Maintenance section.
• CLAUDE.md and GEMINI.md are one-line @AGENTS.md shims in draft docs: complete agent and deploy contributor guidance #92.
• deploy/README.md documents the OpenBao credential
  refresh/degraded-refresh behavior in draft docs: complete agent and deploy contributor guidance #92.
• deploy/README.md points contributors to the canonical authoring rules,
  exact validation command, and roadmap epic Roadmap: declarative GitHub-org-as-code — extend coverage & sustain the source of truth #56 in draft docs: complete agent and deploy contributor guidance #92.
• kubectl kustomize deploy/ > /dev/null and git diff --check pass for
  draft docs: complete agent and deploy contributor guidance #92.

Rough size

Small documentation and instruction-file completion.