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

.cargo/config.toml

@@ -0,0 +1,7 @@
+[target.x86_64-unknown-linux-gnu]
+linker = "clang"
+rustflags = ["-C", "link-arg=-fuse-ld=mold"]
+
+[build]
+rustc-wrapper = "sccache"
+jobs = 8