Fix cmd_apply branch switch-back handling

uspecs/changes/archive/2602/2602251222-pr-apply-switch-back/change.md

@@ -0,0 +1,12 @@
+---
+registered_at: 2026-02-25T11:53:53Z
+change_id: 2602251153-pr-apply-switch-back
+baseline: 3ae1eedce4ac7c85de33e2f621ef820b07b4ea79
+archived_at: 2026-02-25T12:22:03Z
+---
+
+# Change request: conf.sh cmd_apply should switch back to original branch after sending PR
+
+## Why
+
+When `cmd_apply` is called on a PR branch, it needs to switch to the default branch, fast-forward it, and then switch back to the original branch. Currently `cmd_ffdefault` in `pr.sh` switches back to the original branch immediately after fast-forwarding, instead of letting the caller control the return flow.

uspecs/changes/archive/2602/2602251222-pr-apply-switch-back/how.md

@@ -0,0 +1,18 @@
+# How: pr.sh cmd_apply should switch back to original branch after sending PR
+
+## Approach
+
+- Fix `cmd_ffdefault` in `pr.sh`: remove the switch-back block and its cleanup trap so the command
+  stays on the default branch after fast-forwarding. This makes it a clean primitive: switch to
+  default if needed, fetch, fast-forward, done.
+
+- Fix `cmd_apply` in `conf.sh`: move the `prev_branch` capture to before the `ffdefault` call.
+  Currently it is captured after `ffdefault` has already switched back, which is redundant and
+  obscures intent. With `ffdefault` no longer switching back, the correct branch is captured before
+  any switching occurs, and `pr.sh pr --next-branch "$prev_branch"` (already present) handles the
+  final switch-back.
+
+References:
+
+- [uspecs/u/scripts/_lib/pr.sh](../../../../u/scripts/_lib/pr.sh)
+- [uspecs/u/scripts/conf.sh](../../../../u/scripts/conf.sh)

uspecs/changes/archive/2602/2602251222-pr-apply-switch-back/impl.md

@@ -0,0 +1,12 @@
+# Implementation plan: conf.sh cmd_apply should switch back to original branch after sending PR
+
+## Construction
+
+- [x] update: [uspecs/u/scripts/_lib/pr.sh](../../../../u/scripts/_lib/pr.sh)
+  - fix: remove switch-back block (lines 211-215) and its cleanup trap (line 200) from `cmd_ffdefault` so it stays on the default branch after fast-forwarding
+
+- [x] update: [uspecs/u/scripts/conf.sh](../../../../u/scripts/conf.sh)
+  - fix: move `prev_branch` capture in `cmd_apply` to before the `ffdefault` call so the original branch is remembered before any switching occurs
+
+- [x] update: [uspecs/u/scripts/conf.sh](../../../../u/scripts/conf.sh)
+  - add: ERR trap in `cmd_apply` after `ffdefault` succeeds to switch back to `prev_branch` on failure; clear trap after `pr.sh pr` succeeds

uspecs/u/scripts/_lib/pr.sh

@@ -192,12 +192,9 @@ cmd_ffdefault() {
     local current_branch
     current_branch=$(git symbolic-ref --short HEAD)
 
-    local switched=false
     if [[ "$current_branch" != "$default_branch" ]]; then
         echo "Switching from '$current_branch' to '$default_branch'..."
         git checkout "$default_branch"
-        switched=true
-        trap 'git merge --abort 2>/dev/null || true; git checkout "$current_branch"' EXIT
     fi
 
     echo "Fetching $pr_remote/$default_branch..."
@@ -207,12 +204,6 @@ cmd_ffdefault() {
     if ! git merge --ff-only "$pr_remote/$default_branch" 2>&1; then
         error "Cannot fast-forward '$default_branch' to '$pr_remote/$default_branch'. The branches have diverged."
     fi
-
-    if [[ "$switched" == "true" ]]; then
-        trap - EXIT
-        echo "Switching back to '$current_branch'..."
-        git checkout "$current_branch"
-    fi
 }
 
 cmd_pr() {

uspecs/u/scripts/conf.sh

@@ -581,9 +581,12 @@ cmd_apply() {
         error "uspecs is already installed, use update instead"
     fi
 
-    # PR: fast-forward default branch (may update local uspecs.yml)
+    # PR: remember current branch, then fast-forward default branch (may update local uspecs.yml)
+    local prev_branch=""
     if [[ "$pr_flag" == "true" ]]; then
+        prev_branch=$(cd "$project_dir" && git symbolic-ref --short HEAD)
         (cd "$project_dir" && bash "$script_dir/_lib/pr.sh" ffdefault)
+        trap 'git -C "$project_dir" checkout "$prev_branch" 2>/dev/null || true' ERR
     fi
 
     local -A config
@@ -610,11 +613,9 @@ cmd_apply() {
     show_operation_plan "$command_name" "$current_version" "$version" "$commit" "$commit_timestamp" "$plan_invocation_methods_str" "$pr_flag" "$project_dir" "$script_dir"
     confirm_action "$command_name" || return 0
 
-    # PR: capture current branch, then create feature branch
+    # PR: create feature branch
     local branch_name="${command_name}-uspecs-${version_string_branch}"
-    local prev_branch=""
     if [[ "$pr_flag" == "true" ]]; then
-        prev_branch=$(cd "$project_dir" && git symbolic-ref --short HEAD)
         (cd "$project_dir" && bash "$script_dir/_lib/pr.sh" prbranch "$branch_name")
     fi
 
@@ -664,6 +665,7 @@ cmd_apply() {
         # Capture PR info from stderr while showing normal output
         (cd "$project_dir" && bash "$script_dir/_lib/pr.sh" pr --title "$pr_title" --body "$pr_body" \
             --next-branch "$prev_branch" --delete-branch) 2> "$pr_info_file"
+        trap - ERR
 
         # Parse PR info from temp file
         pr_url=$(_grep '^PR_URL=' "$pr_info_file" | cut -d= -f2-)