fix(tools): default AGENT_WORK_ROOT to $HOME/mosaic/agent-work

pr-merge.sh and its regression test hardcoded /home/hermes/agent-work as
the AGENT_WORK_ROOT fallback, baking in one operator's home dir. Any other
user (e.g. jarvis) hit 'mkdir: cannot create directory /home/hermes:
Permission denied' on merge, and --skip-queue-guard did not avoid it (the
mkdir is in the API/tea-error path, not the guard).

Default to ${HOME:-/tmp}/mosaic/agent-work instead: per-user isolated,
kept separate from the framework's ~/.config/mosaic config tree, with a
/tmp last-ditch fallback for unset-HOME contexts (systemd/CI). An explicit
AGENT_WORK_ROOT still overrides. 5 occurrences across 2 files.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

packages/mosaic/framework/tools/git/pr-merge.sh

@@ -128,8 +128,8 @@ PY
 merge_gitea_with_api() {
     local host="$1" api_url token basic_auth body_file raw_code payload
     api_url="https://${host}/api/v1/repos/${OWNER}/${REPO}/pulls/${PR_NUMBER}/merge"
-    mkdir -p "${AGENT_WORK_ROOT:-/home/hermes/agent-work}"
-    body_file=$(mktemp "${AGENT_WORK_ROOT:-/home/hermes/agent-work}/pr-merge-api-response.XXXXXX")
+    mkdir -p "${AGENT_WORK_ROOT:-${HOME:-/tmp}/mosaic/agent-work}"
+    body_file=$(mktemp "${AGENT_WORK_ROOT:-${HOME:-/tmp}/mosaic/agent-work}/pr-merge-api-response.XXXXXX")
     payload='{"Do":"squash"}'
 
     token=$(get_gitea_token "$host" || true)
@@ -214,8 +214,8 @@ case "$PLATFORM" in
         TEA_LOGIN="$(get_gitea_login_for_host "$HOST" || true)"
 
         if [[ -n "$TEA_LOGIN" ]]; then
-            mkdir -p "${AGENT_WORK_ROOT:-/home/hermes/agent-work}"
-            TEA_ERROR_FILE=$(mktemp "${AGENT_WORK_ROOT:-/home/hermes/agent-work}/pr-merge-tea-error.XXXXXX")
+            mkdir -p "${AGENT_WORK_ROOT:-${HOME:-/tmp}/mosaic/agent-work}"
+            TEA_ERROR_FILE=$(mktemp "${AGENT_WORK_ROOT:-${HOME:-/tmp}/mosaic/agent-work}/pr-merge-tea-error.XXXXXX")
             if tea pr merge "$PR_NUMBER" --style squash --repo "$OWNER/$REPO" --login "$TEA_LOGIN" 2> "$TEA_ERROR_FILE"; then
                 rm -f "$TEA_ERROR_FILE"
             elif is_known_tea_empty_identity_failure "$TEA_ERROR_FILE"; then