fix(opencode): scope LLM stall timeout to subagent sessions only

[randomm]

Summary

Gates the LLM stream stall timeout to subagent sessions only (those spawned via the task tool with a parentID). Root/interactive PM sessions are now permanently exempt from the stall detector.

Problem

The 3-minute LLM stall timeout in processor.ts fired on ALL sessions indiscriminately. When the PM session called the question tool and waited for human input, no tokens flowed — so the stall detector incorrectly killed the session after 3 minutes.

Fix

• Fetch Session.Info before the stream loop to access session.parentID
• Gate the stall check on session.parentID being defined (line 94): only subagent sessions (spawned via task tool) get the timeout
• Root sessions (parentID === undefined) are permanently exempt
• Introduce StallError typed class so stall errors can be identified and re-thrown after cleanup runs (previously, stall errors were swallowed by the retry catch block)
• Use stallErr save-and-rethrow pattern: stall error saved in catch, all cleanup runs, error re-thrown at end

Files Changed

• packages/opencode/src/session/processor.ts — stall gate + StallError + cleanup-safe propagation
• packages/opencode/test/session/processor-stall.test.ts — tests verifying parentID gating with real Session objects

Fixes #410