fix: return active instead of idle when JSONL entry is null

When readLastJsonlEntry returns null (empty file or read error), getActivityState
now returns "active" instead of "idle" as a conservative fallback. This makes it
consistent with:
- Other fallback cases in the same function (no workspace path, no session file)
- Other agent plugins (Codex, OpenCode, Aider) which return "active" on detection failure
- The stated intent in PR discussion to assume active when process is running

Fixes bugbot comment: https://github.com/ComposioHQ/agent-orchestrator/pull/45#discussion_r2810110669

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

packages/plugins/agent-claude-code/src/index.ts

@@ -551,7 +551,10 @@ function createClaudeCodeAgent(): Agent {
       }
 
       const entry = await readLastJsonlEntry(sessionFile);
-      if (!entry) return "idle";
+      if (!entry) {
+        // Empty file or read error, but process is running - assume active
+        return "active";
+      }
 
       // Check staleness - no activity in 30 seconds means idle
       const ageMs = Date.now() - entry.modifiedAt.getTime();