feat: Classify prose/empty LLM output as ProseOutputError before architecture gate (#1649)

[prompt-driven-github]

Summary

Adds a ProseOutputError classification gate that fires before the architecture conformance check when an LLM returns prose or an empty/whitespace-only response instead of source code. This prevents prose/planning responses from being misreported as architecture-conformance failures.

Closes #1649

Changes Made

Prompts Modified

• pdd/prompts/code_generator_main_python.prompt — Added ProseOutputError(click.UsageError) typed exception and Requirement 5a: prose/empty-output classification gate that fires before architecture/public-surface checks when generated_code_content is blank; includes repair_directive property returning the output-shape instruction; escape hatch via PDD_ALLOW_EMPTY_GENERATION=1
• pdd/prompts/agentic_sync_runner_python.prompt — Added _parse_prose_output_failure helper (checked first, before conformance/surface/churn), limited to 1 prose retry (2 total attempts); added build_prose_output_hard_failure_from_error export; added === generation output extraction failure === hard-failure block

Documentation Updated

• README.md — Added "Prose/empty-output classification gate" bullet; updated "On failure" sentence to mention ProseOutputError prose retry and the new hard-failure block name
• CHANGELOG.md — Added feat bullet for ProseOutputError classification gate, prose retry limit, hard-failure block, and escape hatch

Architecture Metadata

• architecture.json — Synced with ProseOutputError additions (normalized special character encoding)

User Stories

• Policy: warn
• user_stories/story__agentic_sync_runner_code_generator_main.md — issue-derived story linked to: agentic_sync_runner_python.prompt, code_generator_main_python.prompt
• user_stories/contracts/agentic_sync_runner_code_generator_main.contract.md — generated machine-checkable contract
• Validation: ❌ 1 of 1 linked story check(s) failed: user_stories/story__agentic_sync_runner_code_generator_main.md
• ⚠️ Warning (non-blocking): failing linked story check(s): user_stories/story__agentic_sync_runner_code_generator_main.md

Review Checklist

• Prompt syntax is valid
• PDD conventions followed
• Documentation is up to date

Next Steps After Merge

1. Regenerate code from modified prompts in dependency order:

   pdd sync code_generator_main
   pdd sync agentic_sync_runner
   pdd sync one_session_sync
   pdd sync pre_checkup_gate
   pdd sync durable_sync_runner
   pdd sync prompts
   pdd sync sync_main
   pdd sync agentic_sync
   pdd sync checkup
   pdd sync ci_drift_heal
   pdd sync maintenance
   pdd sync agentic_checkup
   pdd sync generate
   pdd sync pin_example_hack
   pdd sync sync_orchestration
   

2. Run tests to verify functionality
3. Deploy if applicable

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Created by pdd change workflow