fix: stop merging sub-agent turns into parent session

[BYK]

Summary

• Sub-agent requests (OpenCode explore/general agents) were being merged into the parent session via x-parent-session-id header lookup, causing the full Lore pipeline to run on the sub-agent's messages using the parent's session state — corrupting gradient state, cache analytics, timing baselines, and model tracking
• Removed the parent-merge priority in identifySession() so sub-agents naturally get independent sessions through their own x-session-affinity header, benefiting from the full pipeline (LTM, gradient, distillation, curation) on their own state
• Cleaned up all now-dead isSubagentTurn guards from handleConversationTurn(), postResponse(), and the structural compaction dispatch

Problem

When OpenCode spawns a sub-agent (e.g. "explore" for file search), the gateway merged it into the parent session, then ran the full pipeline against the parent's state with the sub-agent's completely different system prompt and messages. This caused:

1. False cache divergence alerts — sub-agent body compared against parent's lastRequestBody
2. Gradient state corruption — sub-agent's tiny message array overwrote parent's lastLayer, lastTransformedCount
3. Cache analytics poisoning — sub-agent body stored as lastRequestBody, poisoning next parent comparison
4. Bust rate contamination — sub-agent's always-cold cache stats fed into parent's dynamic context cap
5. Model/protocol corruption — sub-agent's model overwrote parent's lastModel for warmup profile
6. Timing corruption — lastRequestTime reset by sub-agent in getOrCreateSession()

Approach

Sub-agents already carry their own x-session-affinity nanoid. The fix simply removes the x-parent-session-id merge priority in identifySession(), letting sub-agents fall through to the normal Tier 1 path where they get independent sessions. Net deletion: -113 lines.

Files changed

File	Change
packages/gateway/src/pipeline.ts	Remove parent-merge in identifySession(), remove all isSubagentTurn logic, remove structural compaction sub-agent guard
packages/gateway/test/cache-warmer.test.ts	Update simulateGapRecording helper, remove 3 obsolete sub-agent tests