[no bump] Move technical docs to docs/, add icon and PNG export tool

README now carries the icon, TL;DR, description and features; the
technical sections live in docs/. tools/icon_png.py renders
images/icon.svg to a square PNG (512x512 by default, --resolution N).
plugin.json points at the plugin's own icon.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/how-plugins-work.md

@@ -0,0 +1,26 @@
+# How exteraGram plugins work
+
+## How exteraGram plugins actually work
+
+A plugin, as exteraGram loads it, is exactly **one Python file** on the device:
+`/data/user/0/com.exteragram.messenger/files/plugins/<plugin_id>.py`. The distribution
+convention names the same content `<id>.plugin`.
+
+The app reads plugin metadata **statically via AST** — it looks for top-level constant
+assignments (`__id__`, `__name__`, `__description__`, `__author__`, `__version__`,
+`__icon__`, `__app_version__`, `__sdk_version__`, `__requirements__`) without executing the
+file. Only `__id__` and `__name__` are required.
+
+The file must define exactly one class inheriting `BasePlugin` (`from base_plugin import
+BasePlugin`). Lifecycle methods (`on_plugin_load`, `on_plugin_unload`, `create_settings`) and
+hooks (`on_send_message_hook`, `pre_request_hook`, `post_request_hook`, `on_update_hook(s)`,
+`on_app_event`) are all called by the host; hooks must be registered inside
+`on_plugin_load` (e.g. `self.add_on_send_message_hook()`). A hook returns a
+`HookResult(strategy=HookStrategy.MODIFY, params=params)` to apply changes, or a bare
+`HookResult()` to pass through. Settings rows returned from `create_settings` come from
+`ui.settings` (`Header`, `Input`, `Switch`, `Text`, `Selector`, `Divider`) and persist through
+`self.get_setting`/`self.set_setting`. Logging goes through `android_utils.log`; Chaquopy also
+redirects `sys.stdout`/`sys.stderr` to the logcat tags `python.stdout`/`python.stderr`.
+
+This repo does not use the official Elyx/ElyxBuilder archive format — see
+[`sdk-notes.md`](sdk-notes.md) for why.