fix(compaction): respect previous message agent on context injection

[code-yeongyu]

Summary

• Fix hardcoded agent: "general" in injectCompactionContext to respect the actual agent from previous messages

Problem

When compaction context is injected via injectHookMessage, the agent is hardcoded as "general":

const success = injectHookMessage(summarizeCtx.sessionID, prompt, {
  agent: "general",  // ← Always "general", ignoring actual agent
  model: { providerID: summarizeCtx.providerID, modelID: summarizeCtx.modelID },
  path: { cwd: summarizeCtx.directory },
});

This causes the compaction context message to be attributed to "general" agent instead of the actual agent (e.g., "Sisyphus", "oracle") that was active when compaction was triggered.

Solution

Remove the hardcoded agent: "general" and let injectHookMessage's existing fallback logic handle it:

// Line 192 in injectHookMessage already has:
const resolvedAgent = originalMessage.agent ?? fallback?.agent ?? "general";

By not passing an explicit agent, injectHookMessage will:

1. Check originalMessage.agent (undefined)
2. Fall back to findNearestMessageWithFields to get the actual agent from stored messages
3. Only use "general" as a last resort

This is a 1-line fix that leverages existing fallback logic.

Testing

• Build passes (bun run build)
• No breaking changes to existing interfaces

[nexxeln]

this is great thanks!