docs: add AGENTS.md for future agent sessions

Compact onboarding file capturing hard-won, non-obvious repo facts:

- Dual-target build (desktop + web/WebGPU) and the wasm-only
  web_sys_unstable_apis rustflag (.cargo/config.toml) that is a no-op
  on desktop — easy to break during cleanup.
- Always run --release for visual checks; the debug ray tracer is too
  slow and will look broken.
- The CPU<->shader mirror in src/physics.rs: it is a hand-maintained
  duplicate of black_hole.wgsl kept solely so the capture/escape
  boundary is testable. Editing one side without the other makes tests
  pass on code the shader contradicts.
- The three Bevy 0.19 gotchas that cause the recurring grey screen
  (nudge_camera for issue #24448, EguiPrimaryContextPass placement,
  real storage buffer asset instead of default handle) — exactly what
  this branch's recent commits fix.
- Reserved-but-intentionally-unwired fields (render_scale, spin) and
  the Rs=1 natural-units convention so an agent does not 'fix'
  intentional dead code or invent unit conversions.
- Git workflow: self-judge whether to commit, granular detailed
  commits, never push.

No code changes.

AGENTS.md

@@ -0,0 +1,65 @@
+# AGENTS.md
+
+A real-time Schwarzschild black-hole renderer in Bevy 0.19. One binary, two targets: desktop and web/WebGPU.
+
+## Build & run
+
+- **Desktop:** `cargo run --release` (debug build is too slow to ray-trace; always use `--release` for visual checks).
+- **Web** (WebGPU only): `trunk serve` → http://127.0.0.1:8080. First-time setup: `cargo install --locked trunk` and `rustup target add wasm32-unknown-unknown`. Release web build: `trunk build --release`.
+- `.cargo/config.toml` sets `--cfg web_sys_unstable_apis` for the `wasm32` target only — required for `web-sys`'s WebGPU bindings. It is a no-op on desktop; do not remove it.
+- `edition = "2024"`. No pinned toolchain file; tested on stable 1.96.
+- `target/` and `dist/` are gitignored. The `dist/` folder may contain a ~200 MB wasm build locally — never commit it.
+
+## Test
+
+- `cargo test`. There is exactly one testable surface: `src/physics.rs` (inline `#[cfg(test)]`) + `tests/physics_test.rs` (integration test via the `singularity_rs::physics` lib export). `src/lib.rs` exists solely to expose `physics` for these tests.
+- The GPU shader is not unit-tested. The whole point of `physics.rs` is to be a CPU mirror that *is* testable.
+
+## Architecture: the CPU ↔ shader mirror
+
+The real renderer is a single full-screen quad running `assets/shaders/black_hole.wgsl` (RK4 geodesic integration + disk/planets/grid/star compositing). `src/physics.rs` is a hand-maintained **CPU mirror** of that integrator, kept only so the capture-vs-escape boundary is unit-testable.
+
+**Changing physics in one place means updating the other.** `bending_accel` / `is_captured` in `physics.rs` must stay in lockstep with the shader's `deriv`/step loop, or the tests will pass on code that the shader contradicts.
+
+Module wiring (entrypoints):
+- `main.rs` — app entry, web fallback gate, plugin wiring (`render::BlackHolePlugin`).
+- `render/plugin.rs` — the `BlackHolePlugin`: spawns the fullscreen quad + `Camera2d`, mirrors params to the GPU uniform each frame.
+- `render/material.rs` — `BlackHoleMaterial` (`Material2d`) + `BlackHoleUniforms` / `SphereData` structs.
+- `camera.rs` — orbit controller (yaw/pitch/zoom) + `WantsPointer` (disables orbit over the UI panel).
+- `params.rs` — `BlackHoleParams` resource, edited live by the egui panel, mirrored into the material each frame.
+- `scene/planets.rs` — `Planet` component + storage-buffer upload.
+- `ui.rs` — egui Controls panel.
+- `web.rs` — wasm-only: WebGPU detection + fallback message.
+
+## Bevy 0.19 gotchas (cause of the recurring "grey screen")
+
+Three things that silently produce a grey/frozen canvas if broken — recent commits on this branch exist precisely to fix these:
+
+1. **`nudge_camera` (render/plugin.rs)** works around Bevy 0.19 issue #24448: a static `Camera2d` stops rendering after the first frame. It oscillates the camera by a sub-pixel amount each frame. Do not remove it expecting a cleanup.
+2. **bevy_egui 0.41** requires UI systems to run in `EguiPrimaryContextPass`, **not** `Update`. Placing `ui_system` in `Update` panics.
+3. **The planets storage buffer must be a real `ShaderBuffer` asset**, not `Handle::default()`. A default handle makes `AsBindGroup` return `RetryNextUpdate` every frame, silently skipping the quad's draw — the screen shows only the camera clear color. The quad is pre-filled with a `MAX_PLANETS`-sized zeroed buffer at startup; `upload_planets` updates it.
+
+When debugging a blank/grey screen, check these three before the shader.
+
+## Conventions
+
+- **Natural units: `Rs = 1`** throughout (Rust + WGSL). `BCRIT = 3√3/2·Rs ≈ 2.598` is a literal in `physics.rs` because `f32::sqrt` isn't `const`; the integration test guards the literal.
+- **`render_scale` and `spin` are reserved, not wired.** `render_scale` does not map to a real sub-resolution target (README documents this); `spin` is Phase 2 (Kerr). Both carry `#[allow(dead_code)]` deliberately — don't treat them as missing work.
+- **`steps` is the real performance/quality lever**, not `render_scale`. Lower it in the Controls panel for FPS.
+- **Web defaults differ** via `cfg!(target_arch = "wasm32")`: `steps` 200 (web) vs 300 (desktop), `render_scale` 0.75 vs 1.0.
+
+## Git workflow
+
+- After finishing a change, **decide for yourself whether it should be committed** — don't stop and ask. Commit when the work forms a coherent, complete unit (a fix builds, tests pass, code compiles); hold off only if it's mid-flight or known-broken.
+- **Commits are granular and detailed.** Split by concern: one logical change per commit, not one giant dump. The message explains *what* and *why* (the gotcha it fixes, the invariant it restores), not just a restatement of the diff.
+- **Never push.** Local commits only; leave pushing to the human.
+
+## Reference docs
+
+- `README.md` — controls, how-it-works, project layout, status.
+- `docs/superpowers/specs/2026-07-09-interstellar-blackhole-design.md` — design spec incl. Phase 2 (Kerr) plan.
+- `docs/superpowers/plans/2026-07-09-interstellar-blackhole-phase1.md` — Phase 1 implementation plan.
+
+## Skills
+
+This repo has Matt Pocock's engineering skills vendored under `.agents/skills/` and pinned in `skills-lock.json` (e.g. `tdd`, `code-review`, `diagnosing-bugs`, `codebase-design`). They are workspace-scoped and load automatically.