cli: add an ssh-wrapping +ssh action (#12582)

Add a drop-in `ssh` wrapper that sets up the remote environment for
Ghostty. Anything not consumed as one of our own flags is forwarded to
the real, wrapped `ssh` binary. It can be used directly (`ghostty +ssh
user@host`), aliased (`alias ssh='ghostty +ssh --'`), or invoked through
Ghostty's shell integration.

Before exec'ing ssh, `+ssh`:

- Forwards Ghostty environment to the remote (`--forward-env`): sets
TERM=xterm-256color and requests SendEnv forwarding of COLORTERM,
TERM_PROGRAM, and TERM_PROGRAM_VERSION.
- Installs Ghostty's terminfo on the remote (`--terminfo`), informed by
our existing `ssh-cache` system and using our internal xterm-ghostty
terminfo representation.

A third flag, `--cache`, controls cache use; `--cache=false` bypasses
both read and write, which is useful for scripting and for debugging
install failures without polluting the cache.

For shell integration, this replaces the per-shell logic (which made up
roughly a third of our shell integration scripts) with a simple wrapper
function that translates GHOSTTY_SHELL_FEATURES into a `ghostty +ssh`
command line.

src/cli/ghostty.zig

@@ -11,6 +11,7 @@ const list_keybinds = @import("list_keybinds.zig");
 const list_themes = @import("list_themes.zig");
 const list_colors = @import("list_colors.zig");
 const list_actions = @import("list_actions.zig");
+const ssh = @import("ssh.zig");
 const ssh_cache = @import("ssh_cache.zig");
 const edit_config = @import("edit_config.zig");
 const show_config = @import("show_config.zig");
@@ -47,6 +48,9 @@ pub const Action = enum {
     /// List keybind actions
     @"list-actions",
 
+    /// Wrap `ssh` to configure Ghostty terminal integration on remote hosts
+    ssh,
+
     /// Manage SSH terminfo cache for automatic remote host setup
     @"ssh-cache",
 
@@ -148,6 +152,7 @@ pub const Action = enum {
             .@"list-colors" => try list_colors.run(alloc),
             .@"list-actions" => try list_actions.run(alloc),
             .@"ssh-cache" => try ssh_cache.run(alloc),
+            .ssh => try ssh.run(alloc),
             .@"edit-config" => try edit_config.run(alloc),
             .@"show-config" => try show_config.run(alloc),
             .@"explain-config" => try explain_config.run(alloc),
@@ -189,6 +194,7 @@ pub const Action = enum {
                 .@"list-colors" => list_colors.Options,
                 .@"list-actions" => list_actions.Options,
                 .@"ssh-cache" => ssh_cache.Options,
+                .ssh => ssh.Options,
                 .@"edit-config" => edit_config.Options,
                 .@"show-config" => show_config.Options,
                 .@"explain-config" => explain_config.Options,