D81 — Agent-neutral workflow: AGENTS.md canonical, plans as GitHub issues

Status: implemented (2026-07-20).

Context. After the open source release (D79) the repo's agent-facing surface was still Claude-only: instructions lived in CLAUDE.md (a filename only Claude Code reads) and the design→implementation handoff in a Claude skill that committed plans to docs/plans/ on main. That flow was broken by D79.d (protected main: landing a plan now takes one PR, deleting it another) and produced docs(plans): noise in the public history; a Claude-only repo was also inconsistent with Cartographer's own multi-provider configurator.

Decision.

a) AGENTS.md is the canonical agent-instructions file (the cross-agent convention read natively by Codex, OpenCode, Kiro and others); CLAUDE.md remains as a symlink to it, so Claude Code keeps working unchanged.

b) Plans move from docs/plans/ files to GitHub issues (label plan, template .github/ISSUE_TEMPLATE/plan.md). The agent-neutral procedure — self-sufficiency test included — lives in CONTRIBUTING.md §Plan issues; .claude/skills/plan-issue shrinks to Claude-side glue (D-number reservation, graphify pointers, gh issue create/view). The implementation PR closes the issue (Closes #<n>); a consumed plan survives as a closed issue instead of a deleted file.

Rationale. Issues give the handoff artifact first-class linkage to the PR, an archive that survives consumption, zero service commits in the public history, and readability from any agent with gh — the file-based flow had none of these once main became protected.