fix(agent-setup, github-app-token): guard against inherited GH_CONFIG_DIR outside AGENT_HOME

Contamination class: a stale GH_CONFIG_DIR inherited from a prior
session or a different agent's workspace caused generate-token.sh to
write .gh-token into a foreign workspace, silently granting that
agent's gh config access to the wrong token.

Three hardening changes:

1. agent-setup/scripts/setup.sh — before deriving GH_CONFIG_DIR from
   AGENT_HOME, warn and unset any inherited value that points outside
   AGENT_HOME. This prevents the contaminated value from leaking into
   the derived path or the dotfile.

2. agent-setup/SKILL.md — correct the sourcing example from `source ~/.env`
   to `source "$AGENT_HOME/.env"` so the dotfile is sourced from the
   documented location (setup.sh writes to $AGENT_HOME/.env, not ~/
   which may differ).

3. github-app-token/scripts/generate-token.sh — (a) add a hard die()
   guard that refuses to write the token when GH_CONFIG_DIR is outside
   AGENT_HOME; (b) pin GH_CONFIG_DIR="$GH_TOKEN_DIR" on the gh auth
   login invocation so it cannot fall back to any inherited config dir.

Verified:
- bash -n passes on both modified scripts
- With GH_CONFIG_DIR=/tmp/someone-elses/.github AGENT_HOME=/tmp/me,
  setup.sh warns + overrides; generate-token.sh dies before writing.
- With GH_CONFIG_DIR unset and a valid AGENT_HOME, behaviour is
  unchanged (token lands in $AGENT_HOME/.github).

Co-Authored-By: Paperclip <noreply@paperclip.ing>

agent-setup/SKILL.md

@@ -17,7 +17,7 @@ Validates the `AGENT_HOME` environment variable, derives `GH_CONFIG_DIR` as `$AG
 
 ```bash
 bash agent-setup/scripts/setup.sh
-source ~/.env
+source "$AGENT_HOME/.env"
 ```
 
 ## Output

agent-setup/scripts/setup.sh

@@ -5,6 +5,12 @@ die() { echo "ERROR: $*" >&2; exit 1; }
 
 [[ -z "${AGENT_HOME:-}" ]] && die "AGENT_HOME is not set"
 
+# Validate: never accept an inherited GH_CONFIG_DIR that points outside AGENT_HOME
+if [[ -n "${GH_CONFIG_DIR:-}" && "$GH_CONFIG_DIR" != "$AGENT_HOME"* ]]; then
+  echo "WARN: Inherited GH_CONFIG_DIR '$GH_CONFIG_DIR' is outside AGENT_HOME. Overriding." >&2
+  unset GH_CONFIG_DIR
+fi
+
 # 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"

github-app-token/scripts/generate-token.sh

@@ -64,10 +64,15 @@ fi
 mkdir -p "$GH_TOKEN_DIR"
 GH_TOKEN_FILE="$GH_TOKEN_DIR/.gh-token"
 
+# Validate GH_CONFIG_DIR is inside AGENT_HOME (prevents writing the token to a foreign workspace)
+if [[ -n "${GH_CONFIG_DIR:-}" && -n "${AGENT_HOME:-}" && "$GH_CONFIG_DIR" != "$AGENT_HOME"* ]]; then
+  die "GH_CONFIG_DIR '$GH_CONFIG_DIR' is outside AGENT_HOME '${AGENT_HOME}'. Refusing to write token to a foreign workspace."
+fi
+
 printf '%s' "$TOKEN" > "$GH_TOKEN_FILE"
 chmod 600 "$GH_TOKEN_FILE"
 
 # --- Authenticate gh CLI ---
-gh auth login --with-token < "$GH_TOKEN_FILE"
+GH_CONFIG_DIR="$GH_TOKEN_DIR" gh auth login --with-token < "$GH_TOKEN_FILE"
 
 echo "Authenticated. Token written to $GH_TOKEN_FILE (expires in 1 hour)."