feat(memory): make consolidate ratio configurable

[subalkum]

Summary #3270

• Added consolidationRatio config option to AgentDefaults (default: 0.5)
• Range: 0.1 to 0.95 to prevent edge cases
• Values > 0.5 = more fidelity preserved (e.g., 0.7 = 70% retention)
• Values < 0.5 = more aggressive compression

Context

Per @chengyongru feedback:

• Trigger point (token threshold) remains hardcoded as it's a reliability engineering decision
• Consolidation ratio is now configurable as it's a user preference dependent on use case
• Time-based compaction unchanged (already configurable)

this time the pr is clean haha :)