feat(session): add LLM stream stall detector in processor.ts

[randomm]

Motivation

When an LLM provider stalls mid-stream (no tokens for minutes), the subagent appears "running" but is actually frozen. The for-await loop in processor.ts hangs indefinitely waiting for the next SSE chunk. The abort signal is only checked after a token arrives — if no token arrives, the check never runs. The only safety net is a 30-minute timeout, meaning zombie agents waste up to 30 minutes before the system notices.

Proposed Solution

Add a lastTokenTime timestamp that updates on every text-delta, reasoning-delta, and tool-call event in the processor stream loop. At the start of each iteration (after the existing input.abort.throwIfAborted() check at line 56), check Date.now() - lastTokenTime against a configurable stall timeout (default 3 minutes). If exceeded, throw an error: "LLM stream stalled: no tokens received for 3 minutes".

Key hook locations in processor.ts:

• Line 56: stall check goes after input.abort.throwIfAborted()
• Line 81: case "reasoning-delta" — update lastTokenTime = Date.now()
• Line 134-264: tool-call handlers — update lastTokenTime (tool calls indicate LLM is active)
• Line 338: case "text-delta" — update lastTokenTime = Date.now()

Important: This will NOT gracefully abort the hung HTTP connection (SSE reads cannot be interrupted mid-byte). It will however surface the problem — the session fails visibly instead of hanging indefinitely, and Pulse/check_task can take corrective action.

Quality Gates (Non-Negotiable)

• TDD: Write tests before implementation
• Coverage: 80%+ test coverage for new code
• Linting: All code passes project linting rules
• Local Verification: All tests pass locally before completion

Acceptance Criteria

• lastTokenTime tracked per session in the processor stream loop
• Stall timeout configurable (default 3 minutes, overridable via env var OPENCODE_STALL_TIMEOUT_MS)
• On stall detection: descriptive error thrown, session status set to "failed"
• Existing tests still pass
• New test for stall detection (mock stream that delivers tokens then goes silent)
• Log instrumentation: warn-level log on stall detection with session ID and elapsed time

Definition of Done

• Tests written and passing
• Typecheck passes
• Linting passes

Fork Manifest Requirement

This issue modifies the subagent monitoring system introduced by the async-tasks fork feature. Upon completion, update .fork-features/manifest.json entry async-tasks:

• modifiedFiles: Add packages/opencode/src/session/processor.ts
• criticalCode: Add lastTokenTime, OPENCODE_STALL_TIMEOUT_MS, LLM stream stalled
• absorptionSignals: Add stall.*detector, stream.*stall, lastTokenTime

This ensures sync-time agents understand the stall detection logic and can verify it survives upstream merges.