feat: add E2E classification step and prefer appending tests (#642)

[prompt-driven-github]

Summary

Prevents unnecessary test file accumulation by adding a lightweight E2E classification step (Step 8.5) that skips expensive E2E test generation when unit tests already provide sufficient coverage, and by making Step 6 prefer appending tests to existing test files by default.

Closes #642

Changes Made

New Prompt

• prompts/agentic_bug_step8_5_e2e_classification_LLM.prompt — Lightweight classification prompt (follows Step 5.5 pattern) that outputs E2E_NEEDED: yes/no before Step 9 runs

Prompts Modified

• prompts/agentic_bug_step6_test_plan_LLM.prompt — Make appending to existing test files the preferred default; only create new files when no relevant test file exists
• prompts/agentic_bug_orchestrator_python.prompt — Add Step 8.5 to step sequence, context accumulation, timeouts, and skip logic for Step 9
• prompts/agentic_bug_python.prompt — Update description from "10-step" to "multi-step", add pdd-interface/pdd-reason metadata
• prompts/agentic_bug_step8_verify_LLM.prompt — Update step count reference
• prompts/agentic_bug_step9_e2e_test_LLM.prompt — Update step count, note conditional execution
• prompts/agentic_bug_step10_pr_LLM.prompt — Update step count reference

Documentation Updated

• README.md — Add Step 8.5 description, update Step 6 and Step 9 descriptions

Review Checklist

• Prompt syntax is valid
• PDD conventions followed (pdd-reason, pdd-interface, pdd-dependency tags)
• Step 8.5 follows the established Step 5.5 classification pattern
• Documentation is up to date

Next Steps After Merge

1. Regenerate code from modified prompts in dependency order:

   pdd sync agentic_bug
   pdd sync agentic_bug_orchestrator

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

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Created by pdd change workflow