Pass a custom template and mixin kits through to sbx

Sandboxes ignore the host ~/.claude by design: the agent runs as a separate
user with HOME elsewhere, so even a read-only mount is not picked up. Skills
can be shared with sbx skills import, but plugins carry commands, hooks and
MCP servers that only a custom image can deliver.

Adds --template, --stock-template and a repeatable --kit, persisted per
repository so run and refresh reuse them. AI_SBX_TEMPLATE supplies the default
image, so one custom template can be declared once in the user's mise config
and apply to every repository, with --template overriding it per repository
and --stock-template opting out.

save_config now packs two arrays into one argument list separated by a count,
so it ships with a round-trip test covering empty arrays, values containing
spaces, and the boundary between kits and AWS profiles.

tasks/ai/sbx

@@ -6,6 +6,7 @@ CONFIG_ROOT="${XDG_CONFIG_HOME:-$HOME/.config}/ai-sbx"
 DEFAULT_AGENT="${AI_SBX_AGENT:-codex}"
 DEFAULT_MODE="${AI_SBX_MODE:-clone}"
 DEFAULT_TOKEN_DAYS="${AI_SBX_TOKEN_DAYS:-30}"
+DEFAULT_TEMPLATE="${AI_SBX_TEMPLATE:-}"
 
 # GitHub accepts these as query parameters on the token creation form. A write
 # level implies read, so only the highest level is listed. "workflows" is
@@ -43,6 +44,9 @@ Usage:
 Setup opens a pre-filled GitHub token form in your browser. Use "token" on
 its own to replace an expired or revoked token later.
 
+Set AI_SBX_TEMPLATE in your mise config to reuse one custom image across
+every repository without repeating --template.
+
 Setup options:
   --aws-profile NAME     Host AWS profile to expose inside the sandbox.
                          May be supplied more than once. A trailing
@@ -54,6 +58,12 @@ Setup options:
                          of the repository, mounted read-only.
                          Its commits reach the host through the
                          sandbox-<name> git remote. This is the default.
+  --template REF         Custom sandbox image. Defaults to
+                         AI_SBX_TEMPLATE when set.
+  --stock-template       Ignore AI_SBX_TEMPLATE and use the agent's
+                         stock image.
+  --kit PATH             Mixin kit to apply. May be supplied more
+                         than once.
   --replace              Replace the existing sandbox.
 
 Examples:
@@ -292,13 +302,22 @@ load_config() {
 
     declare -p CONFIG_AWS_PROFILES >/dev/null 2>&1 ||
         CONFIG_AWS_PROFILES=()
+
+    declare -p CONFIG_KITS >/dev/null 2>&1 ||
+        CONFIG_KITS=()
+
+    CONFIG_TEMPLATE="${CONFIG_TEMPLATE:-}"
 }
 
 save_config() {
     local agent="$1"
     local mode="$2"
-    shift 2
-    local -a profiles=("$@")
+    local template="$3"
+    local kit_count="$4"
+    shift 4
+
+    local -a kits=("${@:1:kit_count}")
+    local -a profiles=("${@:kit_count + 1}")
 
     mkdir -p "$REPO_CONFIG_DIR"
     chmod 700 "$CONFIG_ROOT" "$CONFIG_ROOT/repos" "$REPO_CONFIG_DIR" 2>/dev/null || true
@@ -308,6 +327,14 @@ save_config() {
         printf 'CONFIG_SANDBOX=%q\n' "$SANDBOX_NAME"
         printf 'CONFIG_AGENT=%q\n' "$agent"
         printf 'CONFIG_MODE=%q\n' "$mode"
+        printf 'CONFIG_TEMPLATE=%q\n' "$template"
+
+        printf 'CONFIG_KITS=('
+        local kit
+        for kit in ${kits[@]+"${kits[@]}"}; do
+            printf ' %q' "$kit"
+        done
+        printf ' )\n'
 
         printf 'CONFIG_AWS_PROFILES=('
         local profile
@@ -517,6 +544,15 @@ create_sandbox() {
         create_args+=(--clone)
     fi
 
+    if [[ -n "$CONFIG_TEMPLATE" ]]; then
+        create_args+=(--template "$CONFIG_TEMPLATE")
+    fi
+
+    local kit
+    for kit in ${CONFIG_KITS[@]+"${CONFIG_KITS[@]}"}; do
+        create_args+=(--kit "$kit")
+    done
+
     create_args+=(
         "$CONFIG_AGENT"
         "$REPO_ROOT"
@@ -528,8 +564,10 @@ create_sandbox() {
 setup_command() {
     local agent="$DEFAULT_AGENT"
     local mode="$DEFAULT_MODE"
+    local template="$DEFAULT_TEMPLATE"
     local replace=false
     local -a aws_profiles=()
+    local -a kits=()
 
     while (($#)); do
         case "$1" in
@@ -551,6 +589,20 @@ setup_command() {
                 mode="direct"
                 shift
                 ;;
+            --template)
+                (($# >= 2)) || die "--template requires a value"
+                template="$2"
+                shift 2
+                ;;
+            --stock-template)
+                template=""
+                shift
+                ;;
+            --kit)
+                (($# >= 2)) || die "--kit requires a value"
+                kits+=("$2")
+                shift 2
+                ;;
             --replace)
                 replace=true
                 shift
@@ -572,7 +624,14 @@ setup_command() {
         validate_aws_profile "$profile"
     done
 
-    save_config "$agent" "$mode" "${aws_profiles[@]}"
+    local kit
+    for kit in ${kits[@]+"${kits[@]}"}; do
+        [[ -e "$kit" ]] ||
+            die "Kit does not exist: $kit"
+    done
+
+    save_config "$agent" "$mode" "$template" "${#kits[@]}" \
+        ${kits[@]+"${kits[@]}"} ${aws_profiles[@]+"${aws_profiles[@]}"}
 
     if sandbox_exists; then
         if [[ "$replace" == true ]]; then
@@ -656,6 +715,13 @@ status_command() {
     printf 'Agent:      %s\n' "$CONFIG_AGENT"
     printf 'Mode:       %s\n' "$CONFIG_MODE"
 
+    printf 'Template:   %s\n' "${CONFIG_TEMPLATE:-stock}"
+
+    if ((${#CONFIG_KITS[@]})); then
+        printf 'Kits:\n'
+        printf '  %s\n' "${CONFIG_KITS[@]}"
+    fi
+
     printf 'Token days: %s\n' "$DEFAULT_TOKEN_DAYS"
 
     printf 'AWS profiles (host -> sandbox):\n'