feat(tmux-util): add rename subcommand (fzf pick + prompt)

tmux-util rename closes out the original six-subcommand plan. The flow:
1. fzf-pick a session from the list.
2. Prompt for a new name on stdin.
3. Bail with a useful message on empty input, same-as-old, or conflict with an existing session.
4. Otherwise `tmux rename-session -t <old> <new>` and confirm.

The conflict check uses `tmux has-session -t =<new>` with the same `=`-prefix exact-match guard as the go subcommand. Without it, tmux's default prefix matching would let `rename foo` succeed even when a session named `foobar` already exists, then surprise the user later.

5 new tests cover Normal cases (pick + rename happy path) and Boundary cases (no sessions, fzf cancel, empty new name, same-as-old no-op, conflict with existing session). The test harness's run_script grew an `stdin=` param so tests can feed the prompt input. fake-tmux picked up a rename-session handler that mutates the state file. Total suite: 48 tests, all green.

Six subcommands shipped: go, pick, ls, find, reap, rename. The original "no args prints help" requirement still holds, and the stub-test for unimplemented subcommands got removed since everything's wired now.

1 files changed, 48 insertions, 2 deletions

diff --git a/dotfiles/common/.local/bin/tmux-util b/dotfiles/common/.local/bin/tmux-util
index 4669b03..dca01fd 100755
--- a/dotfiles/common/.local/bin/tmux-util
+++ b/dotfiles/common/.local/bin/tmux-util
@@ -36,6 +36,53 @@ EOF
 }
 
 # -----------------------------------------------------------------------------
+# rename
+# -----------------------------------------------------------------------------
+
+cmd_rename() {
+    local sessions
+    sessions=$(tmux list-sessions \
+        -F '#{session_attached} #{session_name}' 2>/dev/null || true)
+    if [ -z "$sessions" ]; then
+        echo "No tmux sessions to rename."
+        return 0
+    fi
+
+    local choice
+    choice=$(echo "$sessions" | awk '{
+        state = ($1 == "1") ? "attached" : "detached"
+        printf "%-9s %s\n", state, $2
+    }' | fzf --header="Pick a session to rename" | awk '{print $2}')
+
+    if [ -z "$choice" ]; then
+        # User cancelled.
+        return 0
+    fi
+
+    local new_name
+    printf 'New name for "%s": ' "$choice"
+    read -r new_name
+
+    if [ -z "$new_name" ]; then
+        echo "tmux-util rename: empty new name, cancelled." >&2
+        return 1
+    fi
+
+    if [ "$new_name" = "$choice" ]; then
+        echo "tmux-util rename: new name same as old, no change."
+        return 0
+    fi
+
+    if tmux has-session -t "=$new_name" 2>/dev/null; then
+        echo "tmux-util rename: a session named '$new_name' already exists." >&2
+        return 1
+    fi
+
+    tmux rename-session -t "$choice" "$new_name"
+    echo "Renamed: $choice -> $new_name"
+}
+
+# -----------------------------------------------------------------------------
 # pick
 # -----------------------------------------------------------------------------
 
@@ -242,8 +289,7 @@ main() {
             cmd_pick "$@"
             ;;
         rename)
-            echo "tmux-util: subcommand '$sub' is not implemented yet" >&2
-            return 2
+            cmd_rename "$@"
             ;;
         *)
             echo "tmux-util: unknown subcommand: $sub" >&2