Add ghostel-initial-input-mode to start in char or line mode

[dakra]

Closes #435.

What

Adds a ghostel-initial-input-mode defcustom (semi-char (default) / char / line) that sets the input mode a freshly started ghostel terminal begins in, so users coming from shell can start directly in line mode without the sleep-for + ghostel-line-mode 'force advice from the issue.

How

• char applies immediately at process start.
• line cannot engage until the shell prints its first prompt to anchor the input region, so ghostel--apply-initial-input-mode arms a buffer-local flag and the existing per-redraw ghostel--line-mode-post-redraw path enters line mode on the first redraw that exposes a real prompt. The new ghostel--line-mode-startup-prompt-ready-p gates on an OSC 133 prompt property or ghostel-prompt-regexp (not the bare cursor), so it skips a blank pre-prompt screen. No timer/sleep.
• A manual mode switch during startup wins (the flag is dropped without entering).
• Scoped to the interactive ghostel entry point, not ghostel-exec.

Testing

• make -j8 all passes (build, Zig, native, elisp, lint).
• 4 new native tests in test/ghostel-line-mode-test.el: deferral-then-entry, char immediate, line arming, manual-override-cancels.
• Live-verified in a real bash session: line defers then enters on the first prompt (~0.5s) with end-to-end editing + send; char enters immediately; semi-char unchanged.