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, 28 insertions, 0 deletions

diff --git a/tests/tmux-util/fake-tmux b/tests/tmux-util/fake-tmux
index b5a1e61..1b84956 100755
--- a/tests/tmux-util/fake-tmux
+++ b/tests/tmux-util/fake-tmux
@@ -151,6 +151,34 @@ case "$cmd" in
     attach-session|switch-client)
         # No state mutation needed — the call log already records intent.
         ;;
+    rename-session)
+        # Forms: rename-session -t <old> <new>   OR   rename-session <new>
+        old=""
+        new=""
+        while [ "$#" -gt 0 ]; do
+            case "$1" in
+                -t) shift; old="$1"; shift ;;
+                *)  new="$1"; shift ;;
+            esac
+        done
+        if [ -z "$old" ] || [ -z "$new" ]; then
+            echo "fake-tmux rename-session: need both -t <old> and <new>" >&2
+            exit 1
+        fi
+        tmp="$STATE.tmp"
+        : > "$tmp"
+        while IFS= read -r line; do
+            [ -n "$line" ] || continue
+            first="${line%% *}"
+            rest="${line#* }"
+            if [ "$first" = "$old" ]; then
+                printf '%s %s\n' "$new" "$rest" >> "$tmp"
+            else
+                printf '%s\n' "$line" >> "$tmp"
+            fi
+        done < "$STATE"
+        mv "$tmp" "$STATE"
+        ;;
     kill-session)
         session=""
         while [ "$#" -gt 0 ]; do