(CODUX-218) Harden base-update response extraction

[numnx]

🧩 Task: Harden base-update response extraction

Automated task PR opened by Code UX for T01.

📋 Summary

Sprint: Sprint 218 · Fix automatic agent update parsing · Task: T01

Harden automatic agent-update response parsing while preserving strict payload validation, retry behavior, non-mutating failures, and synchronized documentation.

🤖 Model & Provider

Provider	Codex CLI
Model	gpt-5.6-sol

⏱️ Timing

Started	2026-07-13 14:25:15 UTC
Finished	2026-07-13 14:31:01 UTC
Duration	5m 45s

📊 Token Usage

Metric	Value
Input tokens	93,691
Cached input tokens	1,327,872
Output tokens	12,722
Total tokens	1,434,285
Invocations	1
Tool calls	24
Active time	5m 16s

Included usage estimate (subscription/local login): $1.51

✅ QA Review

⏳ QA review runs after this PR is opened. Check the task card in the dashboard for the outcome once it lands.

📝 Full Task Prompt

## Objective
Fix `AgentBaseUpdateService` so provider responses with harmless formatting or supported envelopes are parsed compatibly without weakening the exact `instructionMarkdown` contract.

## Scope
- `src/services/agent-base-update-service.ts`
- `tests/backend/services/agent-base-update-service.test.ts`
- `src/domain/llm/json-extraction.ts` as the existing extractor contract to consume, not redesign
- `AgentBaseUpdateService.parseUpdatePayload` and the `applyUpdate` parser callback

## Implementation Requirements
1. Replace the update-specific direct `JSON.parse` call with `extractJsonFromText`, preserving update-specific `ValidationError` messages for extraction and shape failures.
2. Continue requiring a non-array object with exactly one non-empty string property named `instructionMarkdown`, trimming only the accepted payload value.
3. Preserve selected-preset line-order validation, metadata immutability, and the rule that parsing or safety failures occur before `applyBaseAgentInstructionUpdate`.
4. Add focused coverage for raw JSON, fenced/noisy or supported wrapped JSON, malformed output, empty output, arrays, extra properties, and destructive instruction rewrites.

## Constraints
- Keep the change limited to automatic base-agent update behavior; do not alter routes, persistence schemas, provider runners, generic planning parsing, or the shared extractor algorithm.
- Do not accept arbitrary provider metadata or silently select an unrelated JSON object from the response.
- Preserve structured-provider retry behavior by throwing typed validation errors from the parser.

## Verification
- Run `pnpm exec vitest run tests/backend/services/agent-base-update-service.test.ts tests/backend/domain/llm/json-extraction.test.ts`.
- Run `pnpm run lint`.
- Confirm compatible presentation noise succeeds, invalid or destructive responses do not write the preset, and existing raw JSON behavior remains unchanged.

🌿 Branch Info

Base: feature/CODUX-218-fix-automatic-agent-update-parsing
Head: task/feature-codux-21-t01-codex-ca06158f-mrjbdy2i

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🤖 Generated by Code UX