doc: add extension options panel design spec

Design for an options panel reachable from about:addons, configuring a
context-menu entry on selected text and an Alt+Shift+G shortcut. Adds
storage and menus permissions only, both in Firefox's no-prompt class, so
the install dialog gains no lines and data_collection_permissions stays
"none". The gpt <query> search provider is deliberately untouched.

Technical claims were verified rather than assumed: MV2 background
type:"module" works from Firefox 112 (tested in 153.0.1), so no background
HTML shim is needed; package.json needs "type":"module" or node --test
fails on Node 22.0-22.6; the ChatGPT model/hints/temporary-chat params are
undocumented and unreliable, so every label is written at hint strength.
q= no longer auto-submits, only prefills.

Also excludes docs/ and test/ from the packaged ZIP. Verified: without
this the spec doc itself shipped inside the extension, and a bare glob
alone leaves empty directory entries behind.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

web-ext-config.mjs

@@ -8,6 +8,12 @@ export default {
     "package.json",
     "package-lock.json",
     "web-ext-config.mjs",
+    // Directories need both the bare name and the glob: the glob alone removes
+    // the files but leaves an empty directory entry in the ZIP.
+    "docs",
+    "docs/**",
+    "test",
+    "test/**",
     "web-ext-artifacts/**",
   ],
   build: {