Make output parsing robust to multi-line JSON and code blocks

[dwilding]

The agent emitted IMPLEMENTATION_RESULT with a multi-line JSON object wrapped in a code block and invented its own fields (charms_modified, claim_tested, etc.) instead of using the required title and body. Two fixes:

1. Parser robustness

The regex now handles:

• Single-line JSON (original format)
• Multi-line JSON
• JSON wrapped in a ```json code block

This is a safety net — the prompt should prevent it, but the parser should not fail if the agent formats the JSON differently.

2. Stronger output contract

The prompt now explicitly states:

• The JSON must have exactly two fields — title and body
• Do not invent other fields (no charms_modified, claim_tested, run_tox_result, etc.)
• The JSON must be on a single line — do not wrap in a code block
• Escape newlines inside body as \n