feat(opencode): add LLM stream stall detection and check_task visibility (#398)

[randomm]

Summary

Implements EPIC #398 — Subagent LLM stall detection and visibility.

#399 — LLM Stream Stall Detector:

• Tracks lastTokenTime in processor.ts stream loop, updated on every token-generating event
• Detects stalls when no tokens received for configurable timeout (default 3 minutes, OPENCODE_STALL_TIMEOUT_MS)
• Validates env var for NaN and non-positive values before entering stream loop
• Exports stalledSessions Set for cross-module stall tracking with proper cleanup on all return paths
• Logs warn-level message with session ID and elapsed time on stall detection

#400 — Enhanced check_task Visibility:

• Extends TaskResult with stallDetected, lastToolCalls, lastActivity for running tasks
• Shows last 3 tool calls with name, status, and ISO timestamp
• lastActivity reflects the most recent tool call timestamp
• stallDetected uses SessionProcessor.stalledSessions.has(id) for reliable detection
• Completed/failed tasks omit visibility fields for backward compatibility

Manifest Update:

• Updated async-tasks feature with processor.ts in modifiedFiles
• Added lastTokenTime, OPENCODE_STALL_TIMEOUT_MS, LLM stream stalled, stalledSessions to criticalCode
• Added stall.*detector, stream.*stall, lastTokenTime, export const stalledSessions to absorptionSignals
• Added processor-stall.test.ts to tests

Fixes #398
Fixes #399
Fixes #400