chore: code cleanup and documentation overhaul

- Remove debug header dump from MITM proxy (was temp debugging code)
- Suppress dead_code warnings for intentional OpenAI compat fields
- Rewrite README with styled mermaid architecture diagrams, full
  feature listing, usage examples, and CLI reference
- Update endpoint-gap-analysis: images implemented, audio only stretch
- Update mitm-interception-status: add request modification and error
  capture components
- Update standalone-ls-todo: add new endpoints to test results
- Zero compiler warnings

docs/mitm-interception-status.md

@@ -45,6 +45,21 @@ Client → Proxy (8741) → Standalone LS (as antigravity-ls user)
    - Forwards HTTP/1.1 requests upstream with real DNS resolution (`dig @8.8.8.8`)
    - Chunked response detection for fast completion
 
+6. **Request modification** (`src/mitm/modify.rs`)
+   - Strips LS system instructions down to `<identity>` block only
+   - Removes stale conversation history (keeps only last user message)
+   - Injects client tools, tool configs, generation params
+   - Injects images as `inlineData` (base64) into user message parts
+   - Injects tool results as `functionResponse` parts
+   - Enables Google Search grounding when requested
+   - Updates `Content-Length` header after body modification
+
+7. **Upstream error capture** (`src/mitm/store.rs`)
+   - Captures Google API error responses (HTTP 400, 429, 500, etc.)
+   - Parses error JSON for message and status fields
+   - Stores in `MitmStore` for immediate forwarding to client
+   - Prevents request hangs on upstream failures
+
 ## What We Tried (Historical)
 
 ### 1. Extension Patch — `detectAndUseProxy` ✅ Still Active
@@ -129,8 +144,8 @@ Last event includes `"finishReason": "STOP"` in the candidate.
 # One-time setup (creates user + iptables rule)
 sudo ./scripts/mitm-redirect.sh install
 
-# Run proxy with standalone LS + MITM
-RUST_LOG=info ./target/release/antigravity-proxy --standalone
+# Run proxy (standalone + MITM are default)
+RUST_LOG=info ./target/release/antigravity-proxy
 
 # Check usage
 curl -s http://localhost:8741/v1/usage | jq .