Replace the implicit tmux_local.yaml auto-discovery with an explicit
`include` array in `.config`. Included files are resolved relative to
the parent config, support `~` expansion and absolute paths, and can
be nested (with circular include protection).
Replace `--local`/`-l` flag with `--config`/`-c` on create, edit,
remove, and prj commands, accepting a target file path.
Add `tx migrate` command to automatically convert v1.x configs by
injecting the legacy tmux_local file as an include entry.
BREAKING CHANGE: tmux_local.yaml is no longer auto-discovered. Run
`tx migrate` to add it as an explicit include. The `--local`/`-l`
flag is removed in favor of `--config`/`-c`.
