Add AGENTS.md and update .gitignore

- Add AGENTS.md with project overview for AI agents
- Update .gitignore to exclude CodeWhale and DeepSeek tooling

AGENTS.md

@@ -0,0 +1,92 @@
+# AGENTS.md — CB-C6s-STU-GSI-Fixes
+
+## Overview
+
+TWRP flashable zip for the **CB-C6s-STU** tablet (Spreadtrum/Unisoc) running a
+GSI. It appends `build.prop` properties, installs a touchscreen IDC config,
+and installs a camera sensor config. All install logic lives in a shell-script
+`update-binary` — no compiled code, no build system.
+
+## Build / Package
+
+```bash
+# Create the flashable zip (excluded from git via .gitignore)
+cd /home/purpl/Repos/CB-C6s-STU-GSI-Fixes
+zip -r ../CB-C6s-STU-GSI-Fixes.zip META-INF/ system/ vendor/
+```
+
+No build dependencies, no lint, no test suite.
+
+## Architecture
+
+```
+zip root
+├── META-INF/com/google/android/
+│   ├── update-binary        ← Entry point: POSIX sh script, executed by TWRP
+│   └── updater-script       ← Dummy placeholder (TWRP ignores it)
+├── system/
+│   ├── build.prop           ← Properties APPENDED (>>) to /system/build.prop
+│   └── usr/idc/
+│       └── goodix_ts.idc    ← Copied to /system/usr/idc/goodix_ts.idc (644)
+└── vendor/etc/
+    └── sensor_config.xml    ← Copied to /vendor/etc/ or /system/vendor/etc/ (644)
+```
+
+### Install flow (update-binary)
+
+1. Locate zip root: `dirname $0` → up 3 levels (`../../..`)
+2. Mount `/system` and `/vendor`
+3. **Append** `system/build.prop` → `/system/build.prop` (backup first to `.bak`)
+4. Copy `goodix_ts.idc` → `/system/usr/idc/`
+5. Copy `sensor_config.xml` → `/vendor/etc/` (fallback to `/system/vendor/etc/` for SAR)
+6. Report success via `ui_print()` to `/proc/self/fd/$OUTFD`
+
+### Critical constraint: build.prop is APPENDED, never replaced
+
+`update-binary` line ~72: `cat "$BUILDPROP_SRC" >> /system/build.prop`
+
+The `>>` is intentional. Using `>` or `package_extract_dir` would clobber the
+device's existing properties. Re-flashing duplicates entries — warn the user or
+add idempotency if this becomes a problem.
+
+## Key Files
+
+| File | Role |
+|------|------|
+| `META-INF/…/update-binary` | Entire install logic (shell script) |
+| `META-INF/…/updater-script` | Placeholder; not executed |
+| `system/build.prop` | 5 properties appended to device's build.prop |
+| `system/usr/idc/goodix_ts.idc` | Touchscreen input device config (orientation + type) |
+| `vendor/etc/sensor_config.xml` | Camera module mapping: hi846, gc8034 (rear/front, 4 combos) |
+| `README.md` | User-facing documentation |
+| `.gitignore` | Excludes `*.zip` |
+
+## Coding Conventions
+
+- **Shell**: POSIX sh, no bashisms. Shebang `#!/sbin/sh` (TWRP environment).
+- **Error handling**: `abort()` helper exits with message. `2>/dev/null` on
+  non-critical operations (mount, mkdir, cp backup).
+- **Naming**: `UPPER_CASE` for globals/file paths, `snake_case` for helpers.
+- **Partition logic**: `/vendor` mount is best-effort; falls back to
+  `/system/vendor/etc/` when `/vendor/etc/` doesn't exist (SAR compatibility).
+
+## Git Workflow
+
+- Branch: `main` (root commit only)
+- Commit style: concise summary line, then bullet list of changes
+- `.gitignore`: excludes `*.zip` (the build artifact) and tooling directories
+
+## Tips for AI Agents
+
+- **This is not a software project** — there is no build system, no package
+  manager, no test runner. Changes are edits to shell script or config files.
+- **To add a new fix**: add the payload file under `system/` or `vendor/`,
+  add a step to `update-binary` with `ui_print` and `cp` (or append logic),
+  update `README.md` "What it fixes" table.
+- **To add a new build.prop property**: append a line to `system/build.prop`.
+  If it needs to be idempotent, add a grep guard in `update-binary`.
+- **Testing**: flash the zip in TWRP on the device. There is no emulation.
+- **The `updater-script` is a dummy** — do not add Edify commands there; put
+  all logic in `update-binary`.
+- **Zip path**: the build artifact is written to the repo's parent directory,
+  not inside the workspace. `*.zip` in `.gitignore` covers both locations.