Then build a supervisor that reconciles running tmux sessions against a
declared set.

libtmux drives a real tmux server from code. It gives you objects for the
server, its sessions, windows and panes. Documentation for every language
is at https://libtmux.org/en/.

Language:   C++
Package:    libtmux-cxx (its repository)
Repository: https://github.com/libtmux/libtmux-cxx

Read these before writing code:
- https://libtmux.org/en/cxx/latest/llms.txt
  every page for C++, as a list
- https://libtmux.org/en/cxx/latest/docs.json
  the same list with headings, as JSON
- https://libtmux.org/en/cxx/latest/reference/
  the C++ API reference
If you cannot fetch URLs, clone the repository above and read its README
and examples instead, or ask me to paste those pages in.

Set up first:
1. Report the tmux version with `tmux -V`. libtmux drives a real tmux, so
   nothing below works without one. Stop and tell me if it is missing.
2. If this directory is not a C++ project yet, write a CMakeLists.txt
   declaring cmake_minimum_required and project(), targeting C++23.
   Skip this in a project that already exists, and match what is here.
3. Add the dependency. libtmux-cxx is not published to its registry yet, so
   this installs from the repository at tag v0.1.0-alpha.11.
       include(FetchContent)
       FetchContent_Declare(libtmux-cxx
         GIT_REPOSITORY https://github.com/libtmux/libtmux-cxx
         GIT_TAG v0.1.0-alpha.11)
       FetchContent_MakeAvailable(libtmux-cxx)
   Check https://github.com/libtmux/libtmux-cxx for a newer version, and
   prefer a stable release over this one if there now is any.
4. Write the smallest program that proves the install works: connect to a
   tmux server, create a session, confirm it is there, then kill it. Give
   that server a socket of its own rather than using the default one, so
   this never touches a tmux I am already running. Run it, and show me the
   output and the files you added.

Rules for everything below:
- Use the reference above rather than guessing names. The ten libtmux
  ports do not share spellings, and a name from another language will look
  plausible and not exist.
- If the reference does not have something you expected, say so instead of
  writing a call that does not exist.
- Follow the conventions already in this repository.

Session supervisor

What it does:
- Reads a declared set of sessions and windows from a file.
- Compares it against what the server is running and reports the difference
  before changing anything.
- Applies the difference: creates what is missing, and leaves alone anything
  already correct.
- Has a dry-run mode that prints the plan and touches nothing.

Constraints:
- Converge. Running it repeatedly against an unchanged server must do nothing
  the second time and say so.
- Never kill a window it did not create unless I pass an explicit flag.
- Compare against the server's own state rather than against a file this tool
  wrote last time.
- Report what changed in terms I can read, not a diff of internal structures.

Anchor it in these pages:
- https://libtmux.org/en/tmux/concepts/workspaces/
  workspaces
- https://libtmux.org/en/tmux/concepts/queries/
  queries
- https://libtmux.org/en/tmux/guides/querying-and-filtering/
  querying and filtering