docs(agents): adopt AGENTS.md as canonical AI brief; restructure into TOC + sub-files

[ChrisonSimtian]

Closes #236.

Summary

• New `AGENTS.md` at the repo root — canonical brief for all AI coding tools (Claude Code, Copilot, Cursor, Aider, Codex, etc.)
• Three sub-files under `docs/agents/` for the deeper procedural content (repository layout, release & versioning incl. the PR-creation flow, conventions)
• `CLAUDE.md` becomes a 5-line pointer using Claude Code's `@./AGENTS.md` import syntax — no content duplication
• `.github/copilot-instructions.md` is a thin pointer with the top rules inlined plus a URL to `AGENTS.md` (Copilot doesn't support import syntax, so users get a shorter brief than Claude users — by design per discussion)
• `CONTRIBUTING.md` and `docs/architecture.md` updated to reference `AGENTS.md` instead of `CLAUDE.md`

Why

Per #236: multiple maintainers use multiple AI tools. Each tool reads from a different file (`CLAUDE.md`, `.github/copilot-instructions.md`, `AGENTS.md` for Codex/Aider). The previous `CLAUDE.md` was already 95% tool-neutral content; promoting it to the community-convention `AGENTS.md` location means any tool that supports the convention finds the brief automatically, and the tool-specific files become thin pointers rather than copies.

Structure

• AGENTS.md (~70 lines, must-read top): project identity, stack, commands, 6 critical rules, TOC of sub-files and existing docs
• docs/agents/repository-layout.md: directory structure, project groupings, transition-shim strategy
• docs/agents/release-and-versioning.md: branching, semver policy, PR-creation flow (the procedural section reviewers enforce), release pipeline, NuGet first-publish gotcha
• docs/agents/conventions.md: conventions to respect, what-not-to-do, tool-wrapper recipe (consolidated from three sections that belong together)
• CLAUDE.md: 5-line pointer (`@./AGENTS.md`)
• .github/copilot-instructions.md: thin pointer with inlined top rules + URL

What's NOT in this PR

• Cursor / Aider / Continue tool-specific files — deferred to whenever someone actually uses them (or Project-local AI tooling config (MCP servers + equivalents) checked into the repo #240 - project-local AI tooling config)
• The data-handling policy file (Data-handling policy: what AI tools should not be given access to #239), architecture-brief-for-agents (Architecture brief for AI agents — current and future state of Fallout #237), and AI-contribution-conventions RFC (RFC: AI contribution conventions — PRs, commits, comments, attribution #238) — separate issues, each gets its own sub-file under `docs/agents/` as it lands
• CI drift check — Chris explicitly chose the thin-pointer pattern for Copilot, so byte-equivalence between AGENTS.md and copilot-instructions.md is not a goal

Test plan

• Files render correctly on GitHub (preview after open)
• CI green (docs-only PR, only the `ubuntu-latest-docs` companion fires)
• Quick smoke: open Claude Code on this branch and confirm AGENTS.md content shows up in the session brief (via the `@./AGENTS.md` import in CLAUDE.md)