[BUG] Sub-agent sessions cause gateway blocking — before_prompt_build hooks starve other sessions

[jlin53882]

Issue #601 — Sub-agent sessions cause gateway blocking

Problem

When a sub-agent session (sessionKey contains :subagent:, e.g., agent:main:channel:123:subagent:def456) runs a before_prompt_build hook, the gateway thread is blocked because:

1. auto-recall → LanceDB I/O (embedding + rerank)
2. reflection-injector → LanceDB I/O (loadAgentReflectionSlices)

These run sequentially on the same thread. While the sub-agent is doing I/O, all user sessions wait.

Solution

Skip expensive before_prompt_build hooks for sub-agent sessions — they already inherit full context from the parent agent.

PR

PR #613 — #613

與 PR #516 的關係：為什麼不重疊？

PR	修改目標	Issue
#516	isInvalidAgentIdFormat() agent ID 驗證邏輯	#492
#613	before_prompt_build hooks 新增 subagent skip guard	#601

兩者完全獨立：

• PR fix: complete Issue #492 protection -- per-agent exclusion + internal session guards #516 修改的是 isInvalidAgentIdFormat() 函式（處理 agent ID 格式驗證）
• PR fix: skip before_prompt_build hooks for subagent sessions (#601) #613 修改的是 before_prompt_build hooks 內的 guard（處理 subagent session 跳過邏輯）
• 兩者修改的是完全不同的函式和行號區塊，合併時不會有任何衝突

[jlin53882]

Issue Relationships (Updated)

This issue (#601) was incorrectly marked as duplicate of #492.

Correct relationships:

Issue	Title	PR	Status
#601	Sub-agent sessions cause gateway blocking	#613	Open
#492	Agent ID format validation	#516	Merged
#600	(duplicate of #492)	#516	Merged

Issue #601 is NOT a duplicate of #492 — they are completely different bugs:

• memoryReflection hook in before_prompt_build starves user session (beta.10) #492 / fix: complete Issue #492 protection -- per-agent exclusion + internal session guards #516: isInvalidAgentIdFormat() — agent ID format validation
• [BUG] Sub-agent sessions cause gateway blocking — before_prompt_build hooks starve other sessions #601 / fix: skip before_prompt_build hooks for subagent sessions (#601) #613: before_prompt_build hooks — subagent session blocking

The fix in PR #613 does NOT overlap with PR #516. They modify completely different functions in index.ts.