chore: drop unused provider logos, dead code, placeholder screenshots, and zero-reference planning docs

Removes assets and docs that are zero-references in the codebase and the
GitHub Pages site (scanned with ripgrep across *.swift / *.md / *.html).

Removed assets (no Swift / NSImage / UIImage / Image callers):
- 19 provider logo imagesets in OSGKeyboard/Assets.xcassets/:
  anthropic, apple, ark, codingplanx, cometapi, custom, deepseek,
  gemini, groq, mimo, minimax, mistral, moonshot, openai, openrouter,
  qwen, siliconflow, xai, zhipu
- docs/assets/badges/appstore-{en,zh}.svg (README badges in use are
  ios-{en,zh}.svg and macos-{en,zh}.svg)

Removed dead code:
- OSGKeyboardShared/Models/ProviderLogo.swift (no callers; only maps
  provider ids to the now-deleted logo imagesets)

Removed placeholder screenshots:
- docs/screenshots/6.1/ and docs/screenshots/6.7/ (10 PNGs generated
  by scripts/generate_screenshot_placeholders.py; never replaced with
  real captures, no README/HTML/Swift references)

Removed zero-reference planning / audit md (no code, no README, no
CHANGELOG, no GitHub Pages HTML references):
- docs/ACCOUNT_MANAGED_GATEWAY_TESTING.md
- docs/APPSTORE_METADATA.md
- docs/STOREKIT_TESTING.md
- docs/TYPING_KEYBOARD_BASELINE.md
- docs/ios-pip-voice-session-plan.md
- docs/keyboard-accuracy-plan.md
- docs/keyboard-memory-budget.md
- docs/local-asr-architecture.md
- docs/mac-parity-audit-2026-08-23.md
- docs/personal-dictionary-icloud-kvs-checklist.md
- docs/screenshots/README.md (described the deleted placeholders)

Kept (still referenced):
- Brand marks: OSGBrandMark, OSGLogoWide, OSGStatusMark, osglogo
- docs/TESTING.md (referenced by README.md / README.en.md / CONTRIBUTING.md)
- docs/FLOW_BLUETOOTH_TESTING.md (referenced by docs/TESTING.md)
- docs/RELEASE_NOTES_1.8.0.md (referenced by CHANGELOG.md)
- docs/assets/{badges,whats-new,logo,app-icon,hero-devices} (GitHub Pages)
- AppStoreScreenshots/, marketing/ (App Store /策划素材, not in repo refs)

Updated .gitignore so the deleted md cannot be re-added locally and
pushed back by accident. Asset placeholders, dead code, and unused
badges are also covered by patterns under the same rationale.

No version bump: this is a pure cleanup (chore:) with no user-visible
behavior change.

.gitignore

@@ -112,3 +112,18 @@ CHANGES.md
 .ai-skills-layout-shots/
 .workbuddy-ai/
 reports/
+
+# Internal planning / audit docs kept locally only (zero in-repo or
+# GitHub-Pages references; never re-add these — they were removed
+# in 2026-08 to keep the public docs/ limited to GitHub Pages assets).
+/docs/ACCOUNT_MANAGED_GATEWAY_TESTING.md
+/docs/APPSTORE_METADATA.md
+/docs/STOREKIT_TESTING.md
+/docs/TYPING_KEYBOARD_BASELINE.md
+/docs/ios-pip-voice-session-plan.md
+/docs/keyboard-accuracy-plan.md
+/docs/keyboard-memory-budget.md
+/docs/local-asr-architecture.md
+/docs/mac-parity-audit-2026-08-23.md
+/docs/personal-dictionary-icloud-kvs-checklist.md
+/docs/screenshots/README.md