test: add comprehensive tests for session-parser (19 tests)

[TerminalGravity]

Adds 19 unit tests for src/lib/session-parser.ts — previously had zero test coverage despite being a critical module used by scorecard, timeline, and session-stats tools.

Coverage includes:

• User prompt parsing (string + array content)
• Assistant response parsing with model metadata
• Correction detection ("no", "wrong", "not that", etc.)
• Tool call extraction from assistant tool_use blocks
• Sub-agent spawn detection (Task / dispatch_agent)
• Error event detection from tool_result
• Compaction event detection from system messages
• Summary record parsing (sessionId, branch)
• Malformed JSON line handling (graceful skip)
• Empty file handling
• Epoch timestamp normalization
• Content preview truncation
• Unique ID generation
• findSessionFiles with subagent directory support

All 62 tests pass (43 existing + 19 new).