chore: clean up repo organization

- Standardize on bun: remove package-lock.json, add packageManager field,
  gitignore non-bun lockfiles, fix stray npm install in android:build:clean
- Remove stale build logs and empty dirs (src-tauri/plugins, docs/tickets)
- Move android-dev.sh into scripts/
- Consolidate root docs into docs/ (docker/builder under docs/build/);
  move the architecture overview to docs/architecture/README.md
- Extract Requirements Specification from README into docs/requirements.md
  and slim README down to a project intro + docs index
- Fix internal references to the moved files

.gitignore

@@ -9,6 +9,11 @@ yarn-debug.log*
 yarn-error.log*
 pnpm-debug.log*
 
+# Use bun (see packageManager in package.json); ignore other package managers' lockfiles
+package-lock.json
+yarn.lock
+pnpm-lock.yaml
+
 # Build output
 /build
 /dist