chore - add session validation and periodic reminders

[dpuglielli]

Summary

• Add validateSessionFile() to checkpoint.js - checks for placeholder text before git commit and injects specific warnings
• Add periodic session update reminder every 5 prompts in session-startup.js
• Add comprehensive tests for session validation (10 new tests)
• Redesign statusline with context utilization indicators and session file status
• Add enabledPlugins to settings.json
• Bump memento version 0.3.3 → 0.3.4

Problem Solved

Rules in context weren't being actively consulted at trigger points. Claude would have rules like "update session before commit" but not verify actual session file state.

Solution

1. State validation: Before git commit, read session file and check for placeholder patterns
2. Periodic reminders: Every 5 prompts, prompt Claude to consider if anything worth recording happened
3. Statusline: Show context utilization with color-coded warnings and session file existence

Test plan

• All memento tests pass (42 tests)
• validateSessionFile correctly detects placeholder patterns
• Git commit warning includes specific placeholder text when found
• Manual test: commit with placeholder session file shows STOP warning