AGENTS.md as primary projection target across all tools

[canoo]

Problem

NEXUS currently projects to CLAUDE.md for Claude Code and a separate kiro-nexus-steering.md for Kiro. There is no consistent primary format. When a new tool is added, we invent a new projection target.

Solution

Adopt AGENTS.md as the primary projection format. Both Claude Code and Gemini CLI already recognise AGENTS.md natively as a cross-tool standard (r/ClaudeCode, April 2026). This means a single AGENTS.md projection serves two major tools with zero translation overhead.

Projection hierarchy

.nexus/source.md          (NEXUS-owned canonical source — rich, internal)
    │
    ├── AGENTS.md          → Claude Code, Gemini CLI (native, no conversion)
    ├── .kiro/steering/    → Kiro
    ├── .cursor/rules/     → Cursor (MDC format, via driver #17)
    ├── .windsurfrules     → Windsurf (via driver #17)
    └── .github/copilot-instructions.md → Copilot (via driver #17)

What changes

• setup-nexus.sh writes AGENTS.md at repo root, not just CLAUDE.md
• CLAUDE.md becomes a deprecated alias kept for backwards compatibility
• Core orchestration instructions are consolidated into a single source that AGENTS.md is projected from
• nexus sync (#new) uses this as the primary output format

Dependency

Requires #48 (core/ restructure) to be complete first so the source format is clean before projection begins.

Source: AGENTS.md cross-tool recognition confirmed in Anthropic documentation and r/ClaudeCode community, April 2026.

[unitedideas]

This maps cleanly to AGENTS.md as a projection target, but I would keep one audit step between .nexus/source.md and the generated file.

The cases that usually fail in Claude Code -> Codex moves are not the common prose rules. They are the non-equivalent parts: lifecycle hooks, MCP env/secret references, per-agent restrictions, generated artifacts, and session state. Those probably want a generated validation note next to AGENTS.md instead of being silently flattened into it.

We use that pattern for local harness migration here:
https://bringyour.ai/codex-import-checklist.json

The remote side is no-data; the checklist is just the validation contract an agent can run before trusting the projected context.