feat(action): add --no-focus flag to new-pane

[zh4ngx]

Motivation

Scripted multi-pane workflows (orchestrators that spawn helpers via
zellij action new-pane …) currently steal focus from the interactive
user every time a new pane is spawned. The internal Tab::new_pane
already takes a should_focus_pane: bool, but the action layer hardcodes
true regardless of caller intent — see zellij-server/src/screen.rs
ClientId and TabIndex branches. This PR exposes that switch at the CLI.

Change

Add a --no-focus flag to zellij action new-pane (and zellij action edit for parity, since EditFile follows the same focus path).

Internally, a new should_focus_pane: bool field is added to:

• Action::NewPane (legacy KDL keybind variant)
• Action::NewBlockingPane
• Action::NewFloatingPane
• Action::NewTiledPane
• Action::NewStackedPane
• Action::EditFile

The field is threaded through PtyInstruction::SpawnTerminal and
ScreenInstruction::NewPane, replacing the hardcoded true literals at
screen.rs ClientId and TabIndex branches. The PaneId branch (used when
--near-current-pane resolves to a pane) is unchanged — it already
hardcodes false.

Action::NewInPlacePane is intentionally not threaded: in-place
replaces the focused pane, so "no focus" is semantically meaningless.
The CLI enforces this via clap's conflicts_with(\"in-place\").

Backward compatibility

• CLI default for --no-focus is false, so omitting the flag yields
  the existing focus-stealing behavior.
• All in-tree callsites (KDL parser, plugin APIs, default-plugin keybinds)
  set should_focus_pane: true to preserve current behavior.
• Protobuf fields are added at the next free tag as optional bool —
  older clients deserialize as None, which maps back to true on the
  receiver side. No wire-format break.

Compatibility with related flags

• --no-focus --near-current-pane: allowed. Orthogonal: focus vs.
  placement.
• --no-focus --blocking: allowed. Caller wants to wait for command
  exit but not steal focus from interactive user.
• --no-focus --in-place: rejected at CLI parse time (clap conflict).

Tests added/updated

• New round-trip tests in zellij-utils/src/input/actions.rs:
  • test_new_pane_no_focus_threads_through_tiled
  • test_new_pane_default_focuses_pane
  • test_new_pane_no_focus_threads_through_floating
  • test_new_pane_no_focus_threads_through_stacked
  • test_new_pane_no_focus_threads_through_blocking
  • test_edit_no_focus_threads_through
• zellij-utils/src/ipc/tests/roundtrip_tests.rs fixtures updated.
• zellij-server/src/unit/screen_tests.rs fixtures updated.
• Insta snapshots regenerated for KDL serialization and screen instruction
  debug output.

Test results

• cargo test -p zellij-utils --lib: 384 passed
• cargo test -p zellij-server --lib: 1066 passed
• cargo test -p zellij-client --lib: 111 passed

Verification

$ zellij action new-pane --help | grep -A1 no-focus
        --no-focus
            Spawn the new pane without transferring keyboard focus to it

$ zellij action new-pane --no-focus --in-place
error: The argument '--no-focus' cannot be used with '--in-place'

Out of scope

• Plugin pane variants (NewTiledPluginPane, NewFloatingPluginPane,
  NewInPlacePluginPane) use a separate code path through
  PtyInstruction::FillPluginCwd, which already has its own
  Option<bool> for plugin focus. Threading --no-focus through plugin
  panes would be a larger change touching the plugin-loading flow; not
  covered here. CLI --no-focus only affects terminal panes.