feat(agent-setup): add skill to derive and export GH_CONFIG_DIR from AGENT_HOME

Provides a session dotfile ($AGENT_HOME/.env) that subsequent skills
can source to inherit GH_CONFIG_DIR.

agent-setup/SKILL.md

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+---
+name: agent-setup
+description: Validate AGENT_HOME, derive GH_CONFIG_DIR, and export both to a session dotfile for use by other skills.
+---
+
+# Agent Setup Skill
+
+Validates the `AGENT_HOME` environment variable, derives `GH_CONFIG_DIR` as `$AGENT_HOME/.github`, and exports both to a session dotfile so that child bash sessions and skills invoked in the same session inherit them.
+
+## Required Environment Variables
+
+| Variable | Description |
+|---|---|
+| `AGENT_HOME` | The agent's home directory. Must be an absolute path. |
+
+## Usage
+
+```bash
+bash agent-setup/scripts/setup.sh
+source ~/.env
+```
+
+## Output
+
+- `GH_CONFIG_DIR` is set to `$AGENT_HOME/.github` and exported
+- A dotfile (`~/.env` inside `AGENT_HOME`) is written with `export GH_CONFIG_DIR=...` for session inheritance

agent-setup/scripts/setup.sh

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+#!/usr/bin/env bash
+set -euo pipefail
+
+die() { echo "ERROR: $*" >&2; exit 1; }
+
+[[ -z "${AGENT_HOME:-}" ]] && die "AGENT_HOME is not set"
+
+# Derive GH_CONFIG_DIR — gh stores config at ~/.config/gh by default,
+# so we mirror that structure under AGENT_HOME
+export GH_CONFIG_DIR="$AGENT_HOME/.github"
+
+# Write to a session dotfile so child processes inherit the variables
+mkdir -p "$AGENT_HOME"
+cat > "$AGENT_HOME/.env" <<EOF
+export GH_CONFIG_DIR="$GH_CONFIG_DIR"
+EOF
+
+echo "GH_CONFIG_DIR set to $GH_CONFIG_DIR"