chore(worktree): auto-run yarn install on new worktrees

Add a Claude Code SessionStart hook (matcher: startup) that runs
`corepack yarn install` when node_modules is missing, so worktrees Claude
creates internally are immediately usable. Also extend create-task-worktree.sh
to install deps after the worktree is created, covering Codex/Cursor/manual
flows that share that script.

.claude/hooks/session-start.sh

@@ -0,0 +1,22 @@
+#!/bin/bash
+
+# SessionStart hook: ensure dependencies are installed for the current worktree.
+# Runs `corepack yarn install` when node_modules is missing, so new Claude-created
+# worktrees are usable immediately without a manual install step.
+
+set -u
+
+repo_root="${CLAUDE_PROJECT_DIR:-$(git rev-parse --show-toplevel 2>/dev/null || pwd)}"
+cd "$repo_root" 2>/dev/null || exit 0
+
+# No yarn.lock → not a yarn project or fresh clone; nothing to do.
+[ -f "yarn.lock" ] || exit 0
+
+# node_modules already populated → nothing to do (fast path).
+if [ -d "node_modules" ] && [ -n "$(ls -A node_modules 2>/dev/null | head -1)" ]; then
+  exit 0
+fi
+
+echo "[claude hook] node_modules missing in $repo_root — running corepack yarn install..."
+corepack yarn install
+exit 0

.claude/settings.json

@@ -0,0 +1,16 @@
+{
+  "hooks": {
+    "SessionStart": [
+      {
+        "matcher": "startup",
+        "hooks": [
+          {
+            "type": "command",
+            "command": "\"$CLAUDE_PROJECT_DIR\"/.claude/hooks/session-start.sh",
+            "timeout": 600
+          }
+        ]
+      }
+    ]
+  }
+}