feat: MITM request modification — strip bloat from LLM API requests

Intercepts streamGenerateContent requests and trims:
- System instruction: strips web_application_development, knowledge_discovery,
  persistent_context, skills sections (~18KB saved)
- Content messages: strips empty user_rules, workflows boilerplate,
  conversation summaries (~4.5KB saved)
- Tools: keeps 12 essential coding tools, strips 8 non-essential
  (browser_subagent, generate_image, search_web, etc. ~6KB saved)

Total: ~55% reduction in request size while keeping identity, user info,
and all coding-relevant tools intact. Only modifies 'agent' type requests,
checkpoint requests pass through unmodified.

Also:
- Standalone mode is now the default (use --no-standalone to attach to
  existing LS)
- Enable request modification by default
- Add mold linker, sccache, nextest config (8 thread cap)
- Add .cargo/config.toml and .config/nextest.toml

src/mitm/proxy.rs

@@ -363,7 +363,7 @@ async fn handle_http_over_tls(
     mut client: tokio_rustls::server::TlsStream<TcpStream>,
     domain: &str,
     store: MitmStore,
-    _modify_requests: bool,
+    modify_requests: bool,
 ) -> Result<(), String> {
     let mut tmp = vec![0u8; 32768];
 
@@ -535,12 +535,36 @@ async fn handle_http_over_tls(
 
         // Log LLM calls at info, everything else at debug
         if req_path.contains("streamGenerateContent") {
+            let body_len = request_buf.len() - headers_end;
             info!(
                 domain,
                 req_path = %req_path,
+                body_len,
                 cascade = ?cascade_hint,
                 "MITM: forwarding LLM request"
             );
+
+            // ── Request modification ─────────────────────────────────────
+            // Dechunk body → check if agent request → modify → rechunk
+            if modify_requests && body_len > 0 {
+                let body_slice = &request_buf[headers_end..];
+                let raw_body = super::modify::dechunk(body_slice);
+
+                // Only modify "agent" requests, not "checkpoint" (LS internal)
+                let is_agent = raw_body
+                    .windows(20)
+                    .any(|w| w == b"\"requestType\":\"agent" || w == b"requestType\":\"agent\"");
+
+                if is_agent {
+                    if let Some(modified_body) = super::modify::modify_request(&raw_body) {
+                        // Rebuild request_buf: original headers + rechunked modified body
+                        let new_chunked = super::modify::rechunk(&modified_body);
+                        let mut new_buf = request_buf[..headers_end].to_vec();
+                        new_buf.extend_from_slice(&new_chunked);
+                        request_buf = new_buf;
+                    }
+                }
+            }
         } else {
             debug!(
                 domain,