feat(llm): stamp thinking effort on call-completed log line (#6424)

The performance-analysis pipeline measures harness × provider × model ×
effort from production logs, but thinking effort was the only dimension
not stamped per call — it had to be attributed by config-changeover
timestamp.

Add `thinking_effort = ?cfg.thinking_effort` to the `llm: call
completed` `tracing::info!` in `Llm::complete()` so every call
self-describes all four dimensions. `?`-formatting preserves the
`None`-vs-`Some(...)` distinction the same way the token fields keep
None-vs-zero.

Signed-off-by: Will Pfleger <pfleger.will@gmail.com>
Co-authored-by: Duncan <dcfd242e557282d7a1e2cf2e6877522682f1e5c6156dc92ca7d90eaedd3b0f95@buzz.block.builderlab.xyz>

crates/buzz-agent/src/llm.rs

@@ -226,6 +226,7 @@ impl Llm {
             tracing::info!(
                 model = effective_model,
                 provider = ?cfg.provider,
+                thinking_effort = ?cfg.thinking_effort,
                 duration_ms,
                 input_tokens = ?response.input_tokens,
                 cached_input_tokens = ?response.cached_input_tokens,