feat: create skill variable helper

modules/home/programs/terminal/pi/config/skills/create-skill/SKILL.md

@@ -45,6 +45,7 @@ description: "<One-liner: what it does and when to trigger. Keep under ~200 char
 
 - **Be concise.** Skills are injected into agent context — every line costs tokens. Aim for the minimum needed to reliably guide the agent.
 - **Use scripts for repeatable logic.** If a step involves a multi-line shell command, `jq` pipeline, or API call that won't change between runs, put it in a `.sh` file next to `SKILL.md` and reference it from the workflow. See `address-gh-review/` for an example.
+- **Needs configurable values (paths, identifiers, etc.; not secrets — values are stored as plaintext files)?** Copy `assets/variable.sh` into the new skill's `scripts/` dir as-is. Callers use `variable.sh --get NAME [--require-exec RELPATH]`; the helper prints self-explaining `--set` instructions on "unset" or "set-but-invalid" and exits non-zero, so callers just propagate. The helper self-ignores its `.vars/` store on first `--set`, so no `.gitignore` setup is needed.
 - **Frontmatter is required.** `name` and `description` fields. The description is what the agent uses to decide whether to load the skill, so make it specific about trigger conditions.
 - **Don't over-specify.** Trust the agent to fill gaps. Document the _what_ and _when_, not every micro-step.
 - **Split workflow from reference when the reference surface grows.** If a skill accumulates lookup tables, mapping rules, or capability references that the workflow consults, move them into a sibling `<skill>/<category>/` directory (e.g. `mappings/`, `references/`) with one sub-doc per category and an index `README.md`. Keep `SKILL.md` focused on the hot path — workflow, hard rules, and a short table pointing at the sub-docs. Include a brief style guide in the index README covering (a) defer to authoritative sources (stubs, schemas, generated docs) whenever possible, (b) row/entry formatting conventions, (c) when to create a new sub-doc vs. extend an existing one.

modules/home/programs/terminal/pi/config/skills/create-skill/assets/variable.sh

@@ -0,0 +1,60 @@
+#!/usr/bin/env bash
+# Skill-local variable store. Values live in <skill-dir>/.vars/<NAME>.
+#
+# Usage:
+#   variable.sh --get NAME           # prints value to stdout, exits 0
+#                                    # or prints a self-explaining hint to
+#                                    # stderr and exits 2 if unset.
+#   variable.sh --set NAME VALUE     # writes value, exits 0.
+#
+# Callers should treat a non-zero exit as fatal; the stderr message tells
+# the caller (agent or user) exactly how to populate the missing value.
+
+set -euo pipefail
+
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+SKILL_DIR="$(dirname "$SCRIPT_DIR")"
+STORE="$SKILL_DIR/.vars"
+SELF="$0"
+
+usage() {
+    cat >&2 <<EOF
+Usage:
+  $SELF --get NAME
+  $SELF --set NAME VALUE
+EOF
+    exit 2
+}
+
+case "${1:-}" in
+    --get)
+        [[ $# -eq 2 ]] || usage
+        name="$2"
+        file="$STORE/$name"
+        if [[ ! -f "$file" ]]; then
+            cat >&2 <<EOF
+$SELF: $name is not set.
+Ask the user for the value, then set it:
+  $SELF --set $name <value>
+EOF
+            exit 2
+        fi
+        cat "$file"
+        ;;
+    --set)
+        [[ $# -eq 3 ]] || usage
+        name="$2"; value="$3"
+        [[ "$name" =~ ^[A-Z][A-Z0-9_]*$ ]] || {
+            echo >&2 "$SELF: invalid name '$name' (must match [A-Z][A-Z0-9_]*)"
+            exit 2
+        }
+        mkdir -p "$STORE"
+        # Self-ignore the store so values never get committed, even if the
+        # skill root lacks a .gitignore entry for .vars/.
+        [[ -f "$STORE/.gitignore" ]] || printf '*\n' > "$STORE/.gitignore"
+        printf '%s' "$value" > "$STORE/$name"
+        ;;
+    *)
+        usage
+        ;;
+esac