chore(skills): delegate plan-sprint to SI agent

Sprint planning is SI's job. Skill now spawns SI via Task tool
instead of running the workflow directly.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.claude/skills/plan-sprint/SKILL.md

@@ -7,11 +7,32 @@ description: >
   proposes ticket selection, and writes briefing files per team to
   docs/sprints/sprint-N/.
 user-invocable: true
-allowed-tools: Bash, Read, Grep, Glob, Write, AskUserQuestion
+allowed-tools: Task, Read, Grep, Glob
 ---
 
 # Plan Sprint
 
+**Delegate this entire skill to SI** (Project Manager agent, `subagent_type: si`).
+
+When this skill is invoked, spawn SI using the Task tool:
+
+```
+Task(
+  subagent_type: "si",
+  prompt: "Run /plan-sprint for Sprint N. Read the skill at
+    .claude/skills/plan-sprint/SKILL.md for the full workflow,
+    then execute it. Use the arguments provided: {args}",
+  description: "Plan sprint N"
+)
+```
+
+Pass through any arguments the user provided (e.g. sprint number).
+Present SI's output to the user when done. Do NOT run the workflow yourself.
+
+---
+
+The remainder of this file is SI's reference for executing the workflow.
+
 Generate sprint briefing files for each active team (e.g. `server.md`,
 `client.md`, `copy.md`, `joint.md`) for the next sprint based on current
 project state. Only generate briefings for teams that have tickets in the sprint.