fix: detached new-window/split-window inherit the caller's working directory (tmux parity)

[marhel]

Fixes #99

Summary

A detached new-window or split-window invoked without an explicit -c
inherited the session's original start directory instead of the working
directory of the process that issued the command. tmux uses the caller's
working directory in this case, so running either command from a directory
other than the one the session was created in diverged from tmux.

Both commands are covered here so the issue can be closed as a whole.

Reproduction (Linux, isolated sockets)

SOCK=cwdchk
S=$(mktemp -d); C=$(mktemp -d)               # S = session dir, C = caller dir

( cd "$S" && rmux -L $SOCK new-session  -d -s s0 )
( cd "$C" && rmux -L $SOCK new-window   -d -t s0 -P -F '#{pane_current_path}' )
( cd "$C" && rmux -L $SOCK split-window -d -t s0 -P -F '#{pane_current_path}' )

command	tmux	rmux (before)	rmux (after)
new-window	$C (caller)	$S (session)	$C ✓
split-window	$C (caller)	$S (session)	$C ✓

Verified head-to-head against tmux 3.2a on Linux.

Cause

The CLI forwarded only an explicit -c value and sent None otherwise, so the
server fell back to the session's cwd. new-session already falls back to the
caller's working directory in the same situation — new-window and
split-window simply didn't.

Fix

Fall back to the caller's current working directory when no start directory is
given, mirroring new-session. The server already prefers an explicit start
directory over the session cwd, so no server change is needed.

• Extracted a shared caller_current_working_directory() helper in crate::cli.
• new-window: applied the fallback in run_new_window.
• split-window: applied it in both the target-action and legacy paths, and in
  the request-variant routing condition (the plain SplitWindow request cannot
  carry a start directory, so a resolved cwd now routes through SplitWindowExt).

Tests

A single parametrized regression test (tests/detached_caller_cwd.rs) runs the
same scenario for both subcommands: it creates a session in one directory and
runs the detached command from another, asserting the new pane inherits the
caller's directory. RED before the fix (for the command not yet fixed), GREEN
after.

Docs

Added a row to docs/tmux-compat-decisions.md recording this as ISO required
(rmux must match tmux), backed by the regression test per the doc's CI policy.

Checks

• cargo fmt --all -- --check — clean
• cargo clippy --workspace --all-targets -- -D warnings — clean
• cargo build --workspace — clean
• cargo test --workspace --all-targets — all green except one pre-existing,
  environment-sensitive rmux-server live-attach PTY test
  (live_attach_large_bracketed_paste_survives_irregular_chunks,
  handler_send_keys_tests/input_capture.rs). This change touches zero
  rmux-server files, so that test binary is unaffected by it.

Tested on Linux and Windows.

[shideneyu]

Thanks @marhel — this work was incorporated in adapted form in 7069f01 and shipped in RMUX v0.9.0. The released implementation covers new-window and split-window across the full CLI, tiny CLI and queued/source-file paths. Contributor attribution was preserved.

Thank you again !