feat: support AGENTS.md standard by splitting CLAUDE.md

[ryzizub]

Description

Adopt the AGENTS.md open standard (part of multi-harness epic #66).

Platform-agnostic conventions — philosophy, repository structure, tech-agnostic design, workflow, output directories, key conventions, and guidance — now live in a new root AGENTS.md that tier-2 harnesses (Codex, Gemini CLI, OpenCode) read directly. CLAUDE.md becomes a single @AGENTS.md import followed only by the Claude-Code-specific ## Hooks section, so Claude Code receives the same conventions with nothing duplicated.

Why this approach: Claude Code does not read AGENTS.md natively (mid-2026), but CLAUDE.md supports @path imports. This is Anthropic's documented pattern for the split: single source of truth, zero drift, cross-platform (no symlink, so no Windows admin requirement). Only the Hooks section is Claude-Code-specific and stays behind; everything else moved intact.

Also in this PR:

• Repository Structure section added to AGENTS.md — a commented directory tree covering agents/, hooks/, config/, and skills/, mirroring the Flutter plugin's layout overview.
• Light-touch neutralization so the portable file does not overpromise tier-2 behavior: the auto-recommendation sentence is scoped to Claude Code, the clear-context handoff no longer names /clear, and a note clarifies the /name forms are canonical skill names whose invocation varies per harness.
• markdownlint MD041 exemption for CLAUDE.md: its first line must be the @AGENTS.md import rather than an H1 heading, so MD041 is disabled for the file via an inline directive, matching the repo's PR-template precedent.

Type of Change

• New feature (feat)
• Bug fix (fix)
• Code refactor (refactor)
• Documentation (docs)
• CI change (ci)
• Chore (chore)

Test plan

• Structural gate passes: AGENTS.md exists; CLAUDE.md's first line is exactly @AGENTS.md; the Hooks section stays in CLAUDE.md; no agnostic section is duplicated across the two files; the moved sections are present in AGENTS.md; no hook mechanics leaked into AGENTS.md.
• markdownlint-cli2 and cspell pass on both files with the repo config (CI green).
• Manual: launch a fresh Claude Code session at the repo root, confirm no unresolved-import warning for @AGENTS.md, and confirm the imported conventions (e.g. Key Conventions) appear in context.
• Tier-2 (Codex / Gemini CLI / OpenCode) support is structurally verified, not behaviorally tested — no runner is available in this environment.

Follow-ups (out of scope here)

• Import guardrail: add a small CI/shell assertion that AGENTS.md exists and CLAUDE.md's first line is @AGENTS.md. A rename or typo would otherwise silently empty Claude Code's context while claude plugin validate stays green.
• Epic feat: Phase 0 portability prep for open-standard support #228 ripple: the ~18 agent/skill references that read "the project's CLAUDE.md" of a downstream repo were intentionally left unchanged. As a side effect, running /review on Wingspan itself now reads a slim CLAUDE.md through the Read tool, since the Read tool does not expand the @AGENTS.md import. Teaching those agents to also read a target repo's AGENTS.md belongs to feat: Phase 0 portability prep for open-standard support #228.
• Narrative consistency (under feat: add support for other AI platforms other than Claude Code #66): README.md and CONTRIBUTING.md still frame Wingspan as a Claude Code plugin and do not mention the memory-file import. A later pass should align them.

Closes #225.