Add AGENTS.md with conventions, markdown style, and commit guidelines

AGENTS.md

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+# Agents
+
+## Conventions
+
+- Each build lives in its own directory named `<size>-<vtx>/`
+  (e.g. `3inch-walksnail/`)
+- Betaflight CLI dumps go inside the build directory, named
+  `betaflight_dump.txt`
+- Shared configs (rates, modes, VTX tables) go in the repo root or `radio/` as
+  appropriate
+- README.md is the single source of truth for build specs — keep it in sync when
+  adding files
+
+## README format
+
+Each build section follows this field order:
+
+```
+- **Name:**
+- **Frame:**
+- **Prop size:**
+- **FC:**
+- **ESC:**
+- **BF version:**
+- **Receiver:**
+- **ELRS firmware:**
+- **VTX:**
+- **Motor:**
+- **Weight:**
+```
+
+Use ` - ` (hyphen) as the section separator, not em dash.
+
+## Markdown style
+
+- Wrap lines at 80 columns
+- Tables may exceed 80 columns
+
+## Commit style
+
+- Imperative, sentence case, no period: `Add Lion build and CLI dump`
+- No co-author lines
+- Scope is implicit from the message — no conventional-commit prefixes needed