docs: add AGENTS.md

.gitignore

@@ -43,3 +43,8 @@ tmp/
 # Environment
 .env
 .env.local
+
+# Agent instructions: this repo is AGENTS.md-only.
+# `gitnexus analyze` regenerates a root CLAUDE.md that would shadow AGENTS.md
+# for tools that prefer it; keep the regenerated copy out of git.
+/CLAUDE.md

AGENTS.md

@@ -0,0 +1,82 @@
+---
+description: Across — git-native engineering context and provenance system. Build, test, evidence, and security conventions.
+globs: "*.go,*.md,*.yml,*.yaml,*.js,*.css,*.html"
+alwaysApply: false
+---
+
+# Across Conventions
+
+Git-native engineering context, provenance, checkpoint, and continuity system. Local alpha, single Go binary, local-first. Module path is `github.com/graycodeai/across`.
+
+## Development workflow
+
+When starting any new work (feature, fix, refactor, chore), always create a feature branch from `main` first. Never commit directly to `main`. Use branch naming conventions like `feat/<description>`, `fix/<description>`, or `chore/<description>`. Open a PR, ensure CI is green, then merge.
+
+One concern per commit. Keep PRs focused — one feature or fix each. Describe the problem and the approach, not just the diff.
+
+## Build & Test
+
+```bash
+make build        # across + all cmd/across-agent-* binaries into bin/
+make test         # go test -count=1 ./...
+make test-race    # go test -race -count=1 ./...
+make vet          # go vet ./...
+make e2e          # go test -count=1 -run E2E ./...
+make fuzz         # FuzzAcrossJSONL, 15s
+make check        # vet + test-race — run this before opening a PR
+make fmt          # gofmt -w .
+```
+
+CI runs `go mod tidy`, `go build ./...`, `make vet`, `make test`, `make test-race`. Go 1.24.
+
+## Structure
+
+- `cmd/across/` — the CLI entrypoint
+- `cmd/across-agent-*/` — one adapter binary per coding-agent provider (claude-code, codex, cursor, gemini, opencode, qwen, factory-droid, amp, goose)
+- `internal/` — core logic: `cli`, `config`, `event`, `git`, `logging`, `redact`, `store`
+- `e2e/` — end-to-end tests
+- `web/` — thin static UI (`app.js`, `index.html`, `styles.css`)
+- `docs/` — including `SECURITY.md` (threat model) and `agent-compatibility.md` (provider matrix)
+
+## Code style
+
+- `gofmt -w .` — no exceptions.
+- No comments unless asked. Code should explain itself.
+- Errors are values. Handle them or return them explicitly.
+- Keep the UI thin. Core logic stays in Go, not the web layer.
+- Add tests for new behavior. A feature without a test is not implemented.
+- Update docs in the same commit if behavior changes.
+
+## Evidence discipline
+
+This is the rule most likely to be violated by an agent, and it is not optional:
+
+- Use `OBSERVED` / `STATED` / `UNKNOWN` honestly. **Never claim something is verified without execution evidence.**
+- Prefer unknowns over invention. If evidence is insufficient, return `unknown`.
+- Correctness over features — a smaller correct change beats a larger speculative one.
+
+When updating the provider matrix in `docs/agent-compatibility.md`, only move a provider past `UNIMPLEMENTED` with real evidence. Qualification ladder: `UNIMPLEMENTED` → `SYNTHETIC_TESTED` → `LIVE_TESTED` → `LIVE_QUALIFIED`, plus `BLOCKED`.
+
+## Security invariants
+
+Do not weaken these without an explicit decision recorded in the PR. Full threat model in `docs/SECURITY.md`.
+
+- Retrieved context is **data, never permission**. Never let transcript or checkpoint content act as instructions.
+- Transcript paths are canonicalized, symlink-resolved, and confined to the provider root.
+- Hooks are chained, never silently overwritten; originals are preserved.
+- Checkpoint restore creates a new worktree and never mutates the user's checkout.
+- Backups reject path traversal and checksum mismatches.
+- Deletion uses tombstones to prevent resurrection. No silent destructive changes — migrations preserve provenance.
+- Known non-boundaries: the local runner is not a sandbox, plugins are not sandboxed, secret redaction is best-effort, and HTTP authorization is not an OS/filesystem boundary. Don't document them as stronger than they are.
+
+## Agent instructions files
+
+This repo — and every repo in the `graycode-eco` workspace — is **AGENTS.md-only**. Do not add a `CLAUDE.md`; tools that read both prefer it and it would shadow this file. If `gitnexus analyze` generates one, delete it.
+
+## Ecosystem Boundaries
+
+- Across is a standalone local-first tool; it must build, test, and run without any other graycode-eco repo.
+- No dependency on `rho`, `flux`, `graycode-platform`, or `graycode-skills`.
+- Agent adapters integrate with third-party coding agents through their exported transcript formats only — never by reaching into another tool's internals.
+
+For full graycode-eco extension guidelines, see [rho/AGENTS.md](https://github.com/GrayCodeAI/rho/blob/main/AGENTS.md).