feat(maestro): default daemon TMUX_TMPDIR to $XDG_RUNTIME_DIR (NAP-234)

Plain `tmux ls` in the user's shell now sees maestro-managed sessions
without the `TMUX_TMPDIR=/tmp` workaround, because most shells already
set `TMUX_TMPDIR=$XDG_RUNTIME_DIR` and the daemon now matches.

Precedence: env `MAESTRO_DAEMON_TMUX_TMPDIR` > `[daemon].tmux_tmpdir` >
`dirs::runtime_dir()` > None (tmux's own `/tmp` default). Set
`tmux_tmpdir = "/tmp"` in config.toml if the daemon must outlive the
user's login without linger enabled.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/INSTALL.md

@@ -109,6 +109,23 @@ where you want browser access answer yes — both binaries end up in
 | `--web-exec-start <path>` | Override the web unit's `ExecStart` (default `~/.cargo/bin/maestro-web`). |
 | `-y`, `--yes` | Non-interactive: install binaries + drop systemd unit + reload + enable, but **don't** auto-run `configure` or auto-start the unit (those need real config). Also skips the web step unless an existing `maestro-web.service` is being refreshed. |
 
+## Where tmux sockets land
+
+`maestrod` defaults `TMUX_TMPDIR` to `$XDG_RUNTIME_DIR` (= `/run/user/$UID`)
+so a plain `tmux ls` in your shell — which typically uses the same env var
+— sees maestro-managed sessions. Override in
+`~/.config/maestro/config.toml`:
+
+```toml
+[daemon]
+tmux_tmpdir = "/tmp"
+```
+
+Pick `/tmp` if the daemon must outlive your login session and you can't run
+`loginctl enable-linger` (the runtime dir is wiped on logout, taking the
+tmux sockets with it). The env var `MAESTRO_DAEMON_TMUX_TMPDIR` takes
+precedence over the file.
+
 ## What the systemd unit looks like
 
 The wizard writes a *user* unit (`systemctl --user`), since `maestrod` runs