docs(start-work): document multi-work resume flow in agent template

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/features/builtin-commands/templates/start-work.ts

@@ -16,9 +16,13 @@ export const START_WORK_TEMPLATE = `You are starting a Sisyphus work session.
 2. **Check for active boulder state**: Read \`.sisyphus/boulder.json\` if it exists
 
 3. **Decision logic**:
-   - If \`.sisyphus/boulder.json\` exists AND plan is NOT complete (has unchecked boxes):
-     - **APPEND** current session to session_ids
-     - Continue work on existing plan
+   - If multiple active works are listed in your context:
+     - This means boulder.json has more than one work with status: \`active\` or \`paused\`
+     - Use the Question tool to ask the user which plan to resume
+     - Resume by running \`/start-work {plan-name}\` for the selected plan
+     - If the user says "start a new plan", continue with cold-start auto-selection logic
+   - If exactly one active work is listed and the user did not name a plan:
+     - Auto-resume that single active work
    - If no active plan OR plan is complete:
      - List available plan files
      - If ONE plan: auto-select it