bug(memory): Qdrant collection recreation incomplete for main conversation store

[bug-ops]

Summary

PR #1815 fixed dimension mismatch by recreating Qdrant collections when embedding model changes. However, the fix is incomplete:

Collections recreated:

• zeph_skills ✓
• zeph_mcp_tools ✓

Collections NOT recreated:

• zeph_conversations (main conversation store) ✗
• zeph_sessions (session summaries) ✗
• zeph_graph_entities (graph memory) ✗

Impact

When embedding model changes (e.g., qwen3-embedding 4096-dim → text-embedding-3-small 1536-dim):

• Main conversation semantic search fails
• Session summary storage fails
• Graph entity embeddings fail
• Fallback to SQLite mitigates but degrades performance

Test Case

Run with:

1. Previous session using qwen3-embedding (creates 4096-dim collections)
2. New session using text-embedding-3-small (tries to use 1536-dim vectors with 4096-dim collection)
3. Result: Vector dimension mismatch errors on all Qdrant operations

Error:

Related

• PR bug(qdrant): dimension mismatch crash when switching embedding models with existing collections #1815 (partial fix)
• Session 5 testing (2026-03-17): Qdrant dimension mismatch observed

Expected Behavior

All Qdrant collections should be recreated when:

1. Embedding model changes
2. Dimension mismatch detected on any collection
3. Session startup with mismatched embedding dims

Actual Behavior

Only skill/MCP tool collections recreated. Main conversation collections persist with old dimensions.

Solution

Extend collection recreation logic to cover:

• zeph_conversations
• zeph_sessions
• zeph_graph_entities
• Any other user-data collections

Or: Implement automatic migration when dimension mismatch detected on access.