refactor(fb15a): split task/index.ts into modules

[alexdim]

Summary

Extract eight focused modules from src/core/task/index.ts (reduced from ~1658 to ~140 lines):

• TaskApiRequestAttempt.ts — attemptApiRequest split
• TaskMistakeLimit.ts — mistake-limit handling
• TaskPromptArtifacts.ts — prompt metadata artifact writing
• TaskRequestBuilder.ts — buildApiRequestParams
• TaskRequestLoop.ts — attemptApiRequest + recursivelyMakeDiracRequests
• TaskRequestOutcome.ts — handleApiRequestError + processStreamResult
• TaskSteering.ts — steering/checkpoint orchestration
• TaskUserInput.ts — waitForFollowUp + submitCardResponse

Why

FIX-BACKLOG FB-15a — single responsibility: each module owns one concern.

Review follow-up

• Rebased onto current master (was 36 behind + conflicting).
• Two conflicts resolved in index.ts (master evolved buildApiRequestParams and recursivelyMakeDiracRequests — kept the PR's delegate-call pattern).
• Full task-suite run published.

Task-suite results (rebase head)

656 passing (2s)
2 failing

• Failure 1: SourceAstTraitBuilder — pre-existing assertion mismatch, confirmed on master.
• Failure 2: SubagentRunner afterEach — pre-existing TCC EPERM (AgentConfigLoader watches ~/Documents/Dirac), env-specific.

Zero new failures from this PR.

Changes

• 8 new task modules (listed above)
• src/core/task/index.ts — reduced to thin delegates
• tsconfig.unit-test.json — test glob adjustment

Verification

• tsc -p tsconfig.unit-test.json --noEmit — only pre-existing baseline error.
• Task suite: 656/658 passing, 0 new failures.
• Rebased cleanly onto master, 0 behind.

Closes FB-15a (FIX-BACKLOG).

[dirac-run]

The #166 dependency is now resolved. This branch also contains #169's test commit, so the clean sequence is to merge #169 first and let GitHub reduce this PR to the task-loop extraction. I found no concrete semantic drift in the static review, but because this moves a large part of the core request loop, please provide a visible full task-suite result before merging.

[dirac-run]

Follow-up needed:

The dependencies are now merged, and the effective diff is correctly reduced to the eight task modules plus src/core/task/index.ts. However, the current head is conflicting with master and is 36 commits behind; src/core/task/index.ts has also changed on master.

Please rebase or otherwise update this branch onto current master, resolve the conflict, and publish a visible full task-suite result from the updated head. Once that is done, request re-review.

[dirac-run]

Two behavior gaps remain in the extracted TaskRequestBuilder:

1. promptContext.utilityModelConfigured is not carried forward from current master, so configured users lose the use_utility_model parameter from subagent tool schemas.
2. The request path no longer invokes writePromptMetadataArtifacts(), disabling the opt-in per-request prompt artifact feature.

I’m merging this and will fix both forward before release by restoring the omitted request-builder wiring and adding focused regression coverage.