[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-it-works.md

@@ -0,0 +1,26 @@
+# How Chat Summaries works
+
+## Summarize flow
+
+**Summarize** (⋮ → Plugins → Summarize, or hold the unread button) runs in this order:
+
+1. A bulletin confirms the press, then the history is fetched.
+2. The transcript streams to the configured OpenAI-compatible endpoint.
+3. Only when the first chunk arrives is a summary card injected, directly after the newest
+   analyzed message. If that message is not loaded (an offset, or history scrolled away),
+   the chat is asked to load it first. The chat then jumps to the card once.
+4. Reasoning and answer stream into the card. The finished summary ends with a native
+   `#aisummaries` hashtag and is saved to a local SQLite store, from which it is restored
+   whenever the chat is reopened.
+
+Any failure before step 3 (fetch error, empty history, unreachable host, bad key, HTTP
+error) is reported as a bulletin only: the chat is not modified and the view does not move.
+A failure mid-stream is written onto the card, and a summary saved earlier for the same
+range is kept.
+
+Deleting a card like any other message removes its saved row. The settings page shows how
+many summaries are saved and roughly how much space they take, and offers **Clear all
+summaries** (with a confirmation step).
+
+Summaries exist only on this device, so Telegram's own search, which runs on the server,
+does not find them, including a search for `#aisummaries`.