docs: add AGENTS.md working rules for coding agents

Orientation file so agents do not have to re-derive project conventions
from the 300-line readme every session: environment paths, the honest
red/green state per crate, test-data rules (fixtures in, raw samples
out), git boundaries (feature branches freely, main only on approval),
and a definition of done demanding executed verification over
inspection. Points at readme.md sections rather than duplicating them.

AGENTS.md

@@ -0,0 +1,63 @@
+# AGENTS.md — working rules for coding agents on RustyRPN
+
+readme.md is the single source of truth for design decisions and roadmap.
+Read at minimum these sections before starting work:
+
+- §Workflow — the TDD loop you are expected to follow
+- §General guidelines for AI — hard prohibitions
+- §Data formats — input formats, edge cases, and where test fixtures live
+- §Roadmap and status of functionality — what is green, red, or unstarted
+
+## Environment
+
+- Rust stable (edition 2024, MSRV 1.85) via rustup; on this machine the
+  shims are at `/opt/homebrew/opt/rustup/bin` (in PATH via ~/.zprofile).
+- All cargo commands run from `Application/`:
+  `cargo test`, `cargo fmt --check`, `cargo clippy`.
+- Workspace members: `src/core` (rpn-core, domain logic),
+  `src/cli` (rpn-cli, binary `rpnc`). `src/server` and `src/web` are
+  planned but not scaffolded yet.
+
+## Current state (verify against readme.md roadmap; update both when it changes)
+
+- core/config: implemented, tests green.
+- core/db: tests committed and RED (they reference functions that do not
+  exist yet — the crate does not compile its test target). The first task
+  is the minimal implementation to turn them green. Do not weaken or
+  rewrite these tests to make them pass; they encode agreed behaviour.
+- cli: `fn main() {}`. Command surface specified in Documentation/cli.md.
+- feature/epsilon-parser: branch exists, zero commits. Needs workflow
+  step 2 (spec + test checklist agreed with the maintainer) before any
+  failing test is written.
+
+## Test data
+
+- Real samples: `Application/data/test_input/` — gitignored, never
+  committed, never embed their values in test files or fixtures output.
+- Committed sanitized fixtures: `Application/data/fixtures/`
+  (epsilon / tsdrms / subfranchise). Parser tests should use these.
+  Regenerate with `python3 Application/scripts/sanitize_samples.py`;
+  after any change to fixtures run
+  `python3 Application/scripts/check_fixture_leaks.py` and require exit 0.
+- `Documentation/schema.sql` is the v1 domain schema; it is also the
+  embedded migration (test-asserted in core/db).
+
+## Git rules
+
+- Work on a named feature branch; commit per passing test (workflow step
+  5.3). Commit messages are the project's code documentation: format
+  `area: imperative summary`, body explaining *why*.
+- Never commit or push to `main` without the maintainer's explicit
+  approval. Never rewrite history.
+- Never commit: config files (except config.template.toml), anything under
+  data/ except data/fixtures/, secrets, real customer data.
+
+## Definition of done for a task
+
+1. `cargo test` green (all crates), `cargo fmt --check` and
+   `cargo clippy` clean for files you touched.
+2. Every checklist item from the task's spec verified — by a test or by
+   a command you actually ran, not by inspection alone.
+3. readme.md roadmap updated in the same branch.
+4. A short summary for the maintainer: what changed, what is still red,
+   suggested next step.