feat: multi-line comments and suggestions

[brucearctor]

Context

Both GitLab and GitHub support multi-line inline comments, but we only anchor to a single line.

• GitLab: position.line_range with start.new_line / end.new_line
• GitHub: start_line + line fields in review comments

E9 Concerns

Asking the model for TWO line numbers per finding doubles the hallucination surface. Need validation that end_line >= line and both are within the diff hunk.

Defer until after dogfooding. Ship single-line first. Add multi-line when real usage data shows users want it.

Changes Required

1. Finding.EndLine field in model output
2. ReviewComment.EndLine in VCS types
3. Prompt schema update to request end_line
4. Validation: end_line >= line, both in diff range
5. GitLab: line_range in position
6. GitHub: start_line in review comment
7. Suggestion offset: GitLab suggestion:-N+M syntax

Ref

E9 design session in conversation 8ecd8c13