[FEATURE]: Session loop performance — doom loop in-memory, summary debounce, parallel plugin events #20302

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

Three targeted performance optimizations in the session processing loop:

1. In-memory doom loop detection (session/processor.ts)

Replace MessageV2.parts(ctx.assistantMessage.id) database call with an in-memory toolCallHistory array tracked in ProcessorContext. The doom loop check already has all the data it needs from the streaming event handler — no need to re-query the database on every tool call.

2. Summary debounce (session/processor.ts)

Run SessionSummary.summarize() only on the first step of a session. Subsequent steps produce summaries that overwrite the first one with no additional value, wasting CPU and I/O on redundant summary computation.

3. Parallel plugin event handlers (plugin/index.ts)

Replace sequential for loop with Effect.forEach({ concurrency: "unbounded" }) so slow plugin event handlers don't block the main event stream. Each handler runs in a forked fiber with error isolation via Effect.tryPromise.

Impact

Risk

Minimal — all three changes are surgical and backward compatible:

• Doom loop: same logic, different data source (in-memory vs DB)
• Summary: first step behavior unchanged, subsequent steps skip redundant work
• Plugin handlers: same input, parallel execution with error isolation