Add README with install and usage docs

README.md

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+# pi-edit-last
+
+Edit assistant replies since the last user message in your external editor (Zed, VS Code, etc.).
+ 
+## Installation
+
+```json
+// settings.json
+{
+  "extensions": ["git:github.com:wassname/pi-edit-last@main"]
+}
+```
+
+Or symlink:
+```bash
+ln -s /path/to/pi-edit-last ~/.pi/agent/extensions/edit-last
+```
+
+Then `/reload` in pi.
+
+## Usage
+
+1. Have a conversation with pi
+2. Run `/edit-last-external`
+3. Your `$EDITOR` (Zed) opens with quoted assistant text — edit it
+4. Save and close
+5. Pi receives your edited version as a revision request
+
+## Editor
+
+Uses `$VISUAL` or `$EDITOR` (falls back to `vi`). Set in your shell:
+```bash
+export VISUAL=zed  # or code, vim, etc.
+```