[feature] Emit `AGENTS.md` from spec for cross-host interop

[yuyu04]

What problem does this solve

cladding authors CLAUDE.md to carry project/agent instructions, but that file is read only by Claude-family hosts. The vendor-neutral, Linux-Foundation-stewarded AGENTS.md standard (https://agents.md/) is now read natively by Codex, Cursor, Copilot, Gemini CLI, Aider, Zed, and others (60k+ repos). Since cladding already targets three hosts (Claude Code / Codex / Gemini CLI), its spec-derived context is stranded behind a Claude-only filename — other agents in the same repo don't see it.

Proposed shape

Generate/maintain an AGENTS.md alongside CLAUDE.md, derived from the same sources cladding already owns:

• spec.yaml::project + ai_hints (preferred persona, forbidden/preferred patterns, primary branch, test framework)
• docs/conventions.md (style derived from code)

Emit it during clad init / clad sync so it stays in step. cladding's own AGENTS.md already exists for code-style — this is about generating one for adopting projects from their spec.

Versioning scope (GOVERNANCE.md §2)

• Minor — additive generated artifact (new CLI behavior; doesn't change spec schema)

In-scope check (GOVERNANCE.md §4.1 / §4.2)

• Not regressing Iron Law conformance
• Not bypassing the anti-self-cert guard
• Not forking the Ironclad spec
• Not cosmetic-only — ships with a test asserting the generated AGENTS.md reflects ai_hints/conventions for a fixture project

Alternatives considered

• Symlink AGENTS.md → CLAUDE.md — but the two have different intended audiences/sections; a derived file (spec-sourced, host-neutral) is cleaner and keeps CLAUDE.md as the Claude-specific layer.

Willing to implement?

• Yes — small, high cross-host reach

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From a competitive-gap analysis (portability cluster).

[Inkayuyu04]

Since AGENTS.md is generated from the existing cladding, we recommend that you stop using it.