Default the sandbox agent to claude

The Claude configuration, plugin and skills support added for sandboxes only
applies when the agent is claude, so codex was no longer the sensible default.

tasks/ai/sbx

@@ -3,7 +3,7 @@ set -euo pipefail
 
 PROGRAM="ai:sbx"
 CONFIG_ROOT="${XDG_CONFIG_HOME:-$HOME/.config}/ai-sbx"
-DEFAULT_AGENT="${AI_SBX_AGENT:-codex}"
+DEFAULT_AGENT="${AI_SBX_AGENT:-claude}"
 DEFAULT_MODE="${AI_SBX_MODE:-clone}"
 DEFAULT_TOKEN_DAYS="${AI_SBX_TOKEN_DAYS:-30}"
 DEFAULT_TEMPLATE="${AI_SBX_TEMPLATE:-}"
@@ -79,7 +79,7 @@ Setup options:
                          May be supplied more than once. A trailing
                          -readonly is stripped from the profile name
                          written into the sandbox.
-  --agent NAME           Sandbox agent. Default: codex
+  --agent NAME           Sandbox agent. Default: claude
   --direct               Mount the host working tree read-write.
   --clone                Give the agent a private in-container clone
                          of the repository, mounted read-only.