StorageProvider: Add adversarial markdown tests for history section parsing

[diberry]

Context

Flight's Phase 2 review noted that handles.ts:117-153 uses regex-based section searching for appendHistory(). This is fragile if history.md has unexpected formatting (nested headers, code blocks containing ##).

What to do

Add parser tests covering edge cases:

• Nested headers (### inside ## sections)
• Code blocks containing ## markers
• Empty sections
• Consecutive sections with no content between them

Branch

Work should be done on diberry/sa-phase1-interface

Severity

Nice-to-have (Phase 3 test hardening)

[diberry]

22 adversarial tests added for history parsing — code blocks, empty sections, Unicode, long sections, malformed headers. Shipped on diberry/sa-phase1-interface.