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

src/mitm/store.rs

@@ -68,6 +68,7 @@ pub struct UpstreamError {
     /// HTTP status code from Google (e.g. 400, 429, 500).
     pub status: u16,
     /// Raw error body from Google (usually JSON).
+    #[allow(dead_code)]
     pub body: String,
     /// Parsed error message, if available.
     pub message: Option<String>,