init basic examples and setup

AGENTS.md

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+# AGENTS
+
+## Map
+
+- Root has workspace files.
+- `examples/` has one crate per demo.
+- `docs/hardware/` has board and wiring notes.
+- `memory.x` is shared.
+- `justfile` is source of truth for commands.
+
+## Rules
+
+- Read before edit.
+- Keep fixes small.
+- No broad refactor.
+- No dependency upgrade unless asked.
+- Keep examples independent.
+- Keep demo reliability first.
+- Use simple code.
+- No hidden IDE steps.
+- No fake hardware claims.
+
+## Preserve
+
+- `nix develop`
+- `just fmt`
+- `just check`
+- `just build`
+
+## Hardware
+
+- Blue Pill first.
+- `thumbv7m-none-eabi` first.
+- Do not mix STM32F0 with STM32F103.
+- If hardware detail is unclear:
+  - write the assumption
+  - add a TODO
+  - keep build green
+
+## Style
+
+- Caveman style.
+- Short names.
+- Short comments.
+- Explain embedded idea.
+- Do not explain obvious Rust syntax.
+- No heap.
+- No dynamic dispatch unless teaching needs it.
+- No async before Embassy examples.
+- `unsafe` only if required and justified.
+
+## Commands
+
+- `just build`
+- `just build-blinky`
+- `just build-timer`
+- `just build-button`
+- `just build-embassy-blinky`
+- `just build-embassy-button`
+- `just flash-blinky`
+- `just flash-timer`
+- `just flash-button`
+- `just flash-embassy-blinky`
+- `just flash-embassy-button`
+- `just run-blinky`
+- `just run-embassy-blinky`
+- `just fmt`
+- `just clippy`
+- `just check`
+- `just clean`
+
+## Do Not
+
+- Do not invent board output.
+- Do not claim flash or debug worked on hardware unless tested.
+- Do not hide chip mismatches.
+- Do not add abstractions for one use.