fix(mesh): leave thinking effort to the model's own template

Shared compute pinned BUZZ_AGENT_THINKING_EFFORT. That is not a sampling
hint: mesh translates `reasoning_effort` into the chat template's
`enable_thinking` flag, so any value Buzz sends overrides the default the
model authors shipped — and the right value is model-specific.

Measured against a request captured from the real agent (full ~43k-token
system prompt, real dev-mcp toolset, tool_choice=auto):

  effort   gemma-4-E4B   Qwen3-8B   Qwen3-8B output tokens
  none         0/8          8/8               ~121
  low          4/4          7/8               ~470
  unset        6/6          8/8               ~500

`none` (the previous default, chosen when shared compute was Qwen3-only)
stops gemma tool-calling outright, so its replies were never published.
Enabling thinking instead makes Qwen3 spend ~4x the output budget, which
risks the 4096 cap this preset sets.

Omitting the field leaves `enable_thinking` as None, so mesh passes no
override and each model uses its own template default: 3/3 delivery on
gemma-4-E4B, gemma-4-26B-A4B, gemma-4-31B and Qwen3-8B, and 3/3 replies
through the real buzz-acp + buzz-agent path against a live relay.

Thinking levels are unaffected: an explicit agent/persona/global value is
still preserved and applied.

Signed-off-by: Michael Neale <michael.neale@gmail.com>

crates/buzz-relay/examples/mesh_agent_e2e.rs

@@ -471,10 +471,9 @@ async fn agent_chat_in_isolated_home(
         .env("OPENAI_COMPAT_API_KEY", "buzz-mesh-local")
         .env("OPENAI_COMPAT_API", "chat")
         .env("BUZZ_AGENT_MAX_OUTPUT_TOKENS", "4096")
-        // Must track apply_relay_mesh_env()'s default. `none` suppresses tool
-        // calling on local models, so pinning it here would test a config the
-        // product no longer ships and hide the very regression P5/P6 exist for.
-        .env("BUZZ_AGENT_THINKING_EFFORT", "low")
+        // No BUZZ_AGENT_THINKING_EFFORT: apply_relay_mesh_env() deliberately
+        // leaves it unset so each model's chat template picks its own default.
+        // Pinning a value here would test a config the product does not ship.
         .stdin(Stdio::piped())
         .stdout(Stdio::piped())
         .stderr(Stdio::null());

desktop/src-tauri/src/managed_agents/relay_mesh.rs

@@ -47,14 +47,15 @@ pub fn apply_relay_mesh_env(
     // may deliberately choose a smaller cap or a different effort. This function
     // runs after those layers during readiness, so never clobber their values.
     insert_default_if_unset(env, "BUZZ_AGENT_MAX_OUTPUT_TOKENS", "4096");
-    // `none` suppresses tool calling outright on local models: llama.cpp's
-    // Gemma grammar is lazy under `tool_choice: auto`, and `reasoning_effort:
-    // "none"` keeps the model on its prose path so it never emits the tool-call
-    // token. Measured with the real prompt and toolset, gemma-4-E4B delivered a
-    // reply 0/8 times at `none` and 8/8 at `low`; `minimal` was 3/4. `low` buys
-    // just enough deliberation to pick a tool without spending the output
-    // budget on hidden reasoning.
-    insert_default_if_unset(env, "BUZZ_AGENT_THINKING_EFFORT", "low");
+    // Deliberately no BUZZ_AGENT_THINKING_EFFORT default: mesh translates
+    // `reasoning_effort` into the chat template's `enable_thinking` flag, so any
+    // value we pick overrides each model's own template default — and the right
+    // value is model-specific. Measured with the real prompt and toolset:
+    // gemma-4-E4B delivers 0/8 at `none` but 6/6 with the field absent, while
+    // Qwen3-8B delivers 8/8 either way and burns ~4x the output tokens once
+    // thinking is on (121 -> ~470), risking the 4096 cap. Omitting the field
+    // lets every model use its own default; explicit agent/persona/global
+    // values still apply.
 }
 
 #[cfg(feature = "mesh-llm")]
@@ -107,12 +108,11 @@ mod tests {
             env.get("BUZZ_AGENT_MAX_OUTPUT_TOKENS").map(String::as_str),
             Some("4096")
         );
-        // Must not be "none": that suppresses tool calling on local models, so
-        // the agent answers in prose and the reply is never published.
-        assert_eq!(
-            env.get("BUZZ_AGENT_THINKING_EFFORT").map(String::as_str),
-            Some("low")
-        );
+        // Must stay unset: any value we pick overrides the model's own chat
+        // template default, and the right value is model-specific ("none"
+        // stops gemma tool-calling; enabling thinking makes Qwen3 burn ~4x the
+        // output budget).
+        assert_eq!(env.get("BUZZ_AGENT_THINKING_EFFORT"), None);
         assert_eq!(
             env.get(RELAY_MESH_PREFER_MESH_FOR_AUTO_ENV)
                 .map(String::as_str),
@@ -127,8 +127,6 @@ mod tests {
                 "BUZZ_AGENT_MAX_OUTPUT_TOKENS".to_string(),
                 "2048".to_string(),
             ),
-            // Deliberately not the default ("low"), so this asserts
-            // preservation rather than coinciding with the default.
             ("BUZZ_AGENT_THINKING_EFFORT".to_string(), "high".to_string()),
         ]);
         apply_relay_mesh_env(