Refactor writeReport() — separate concerns into smaller functions

[xhulz]

src/core/reporter.ts — writeReport() has ~128 lines and handles 8 distinct responsibilities:
directory creation, suggestion matching, verdict table, finding blocks, next step text,
forge interpretation, cross-contract section, and file writing.

Break it into smaller named helpers, each independently testable.

Acceptance criteria:

• writeReport() becomes an orchestrator of smaller functions
• Each helper has a unit test
• No change in report output format