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/main.rs

@@ -46,9 +46,9 @@ struct Cli {
     #[arg(long, default_value_t = 8742)]
     mitm_port: u16,
 
-    /// Use a standalone LS (does not touch the real LS)
+    /// Disable standalone LS — attach to the real running LS instead
     #[arg(long)]
-    standalone: bool,
+    no_standalone: bool,
 }
 
 #[tokio::main]
@@ -91,7 +91,7 @@ async fn main() {
     };
 
     // ── Step 2: Backend discovery (or standalone LS spawn) ─────────────────────
-    let standalone_ls = if cli.standalone {
+    let standalone_ls = if !cli.no_standalone {
         // Standalone mode: discover main LS config, spawn our own
         let main_config = match standalone::discover_main_ls_config() {
             Ok(c) => c,
@@ -182,7 +182,7 @@ async fn main() {
                 let ca_pem = ca.ca_pem_path.display().to_string();
                 let config = mitm::proxy::MitmConfig {
                     port: cli.mitm_port,
-                    modify_requests: false,
+                    modify_requests: true,
                 };
                 match mitm::proxy::run(ca, mitm_store.clone(), config).await {
                     Ok((port, handle)) => {
@@ -228,7 +228,7 @@ async fn main() {
 
     // Periodic backend refresh — keeps LS connection details fresh
     // (skip in standalone mode — the port is fixed and discover() would overwrite it)
-    let is_standalone = cli.standalone;
+    let is_standalone = !cli.no_standalone;
     let refresh_backend = Arc::clone(&state.backend);
     let refresh_handle = tokio::spawn(async move {
         if is_standalone {