fix(desktop): stop pinning agents to deprecated SPROUT_ACP_TURN_TIMEOUT

[tlongwell-block]

Why

Desktop-launched agents were being killed at exactly 320s even after PR #566 raised the sprout-acp idle-timeout default to 620s. The reason: the desktop spawn path always emitted the deprecated SPROUT_ACP_TURN_TIMEOUT env var with a hard-coded fallback of 320, which the harness's deprecation handler then used as the effective idle timeout — silently overriding the new default.

What

In desktop/src-tauri/src/managed_agents/runtime.rs, only emit SPROUT_ACP_IDLE_TIMEOUT when the agent record has an explicit idle_timeout_seconds override. When unset, say nothing and let the harness apply its own DEFAULT_IDLE_TIMEOUT_SECS (currently 620s).

Net: -8 / +3 logic lines, plus a doc-comment refresh on DEFAULT_AGENT_TURN_TIMEOUT_SECONDS.

-    if let Some(idle) = record.idle_timeout_seconds {
-        command.env("SPROUT_ACP_IDLE_TIMEOUT", idle.to_string());
-        command.env("SPROUT_ACP_TURN_TIMEOUT", idle.to_string());
-    } else {
-        command.env(
-            "SPROUT_ACP_TURN_TIMEOUT",
-            record.turn_timeout_seconds.to_string(),
-        );
-    }
+    if let Some(idle) = record.idle_timeout_seconds {
+        command.env("SPROUT_ACP_IDLE_TIMEOUT", idle.to_string());
+    }

Effect

• New desktop-launched agents pick up whatever the harness default is. No more drift between desktop and sprout-acp.
• Agents whose users have customized idle_timeout_seconds are unchanged.
• The deprecation warning emitted by sprout-acp when it sees SPROUT_ACP_TURN_TIMEOUT will stop appearing in desktop agent logs.

Notes / Follow-ups

• The legacy turn_timeout_seconds field on ManagedAgentRecord (and its UI exposure in CreateAgentDialog with default "320") is now a dead input on the spawn path. Doc comment on DEFAULT_AGENT_TURN_TIMEOUT_SECONDS updated to reflect that. Removing the field entirely is a separate change — touches DB schema migrations + the dialog UI + the e2e bridge.
• No tests assert on env-var emission (grepped); existing managed_agents tests pass (115/115).

Verification

• cargo build --lib clean
• cargo test --lib managed_agents → 115 passed
• Pre-push lefthook (11 hooks: fmt, check, clippy, tests, tauri-check, web-build, mobile-check, mobile-test, desktop-build) all green