feat(agent-loop): config-driven session resume via RESUME_SESSION_ID

A config agent with `resumeSessionId` set continues a persisted session
instead of starting a fresh `${id}-session-<uuid>`. The id is sourced from
an env var in cordis.yml, so the coding-agent demo can resume a prior
conversation without code changes. The resume is deferred until the
`sessionPersistence` backend loads (via ctx.inject) and is contained: a
missing/unreadable id logs a warning and starts no agent. Adds a real-API
resume e2e proving cross-process continuity through the JSONL backend.

examples/coding-agent/cordis.yml

@@ -61,6 +61,9 @@
     agents:
       - id: main
         model: deepseek-v4-flash
+        # Set RESUME_SESSION_ID to continue a prior persisted session (the ids
+        # live under ./.sessions); unset starts a fresh session each run.
+        resumeSessionId: !!js process.env.RESUME_SESSION_ID
         systemPrompt: |
           You are coding-agent, a CLI coding assistant.