Terminal: new-tab, select-all and close-tab shortcuts

[neo773]

Stacked on #21 (which is itself stacked on #20). The first three commits belong to those PRs and drop out of this diff as they merge. The commit to review here is 8e63b6f.

Chord	Action	Scope
Ctrl+Shift+`	New terminal tab	Global
Cmd/Ctrl+A	Select all, scrollback included	Terminal
Cmd/Ctrl+W	Close active tab	Terminal

New-tab opens the panel when it is closed and adds a tab when it is already open — set_open creates the first tab itself, so doing both would open two. Close-tab goes through the same path as middle-click and the tab's ×, so closing the last one still collapses the drawer and still tells the engine to reap the PTY.

Three things that fail silently if you get them wrong

Bindings must be registered from apply_keymap, not terminal::panel::init. That function clears the keymap, and Shell::new runs it during startup — so anything bound at app init is gone before the first frame. No error; the shortcut simply never fires. There is a test keeping these out of init.

Shifted punctuation is written as the character it produces. ctrl-shift-\`` parses, builds, and then never matches: the platform reports with no shift modifier, so a binding demanding both matches nothing. Letters are the opposite —ctrl-shift-a is right, because Shift stays a modifier. zed's keymaps show the same split (ctrl-, ctrl-?for punctuation;ctrl-shift-c, ctrl-shift-nfor letters). A test pins the chord to key~`, ctrl on, shift off.

macOS "Close Window" moves off ⌘W to ⌘⇧W. AppKit fires menu key equivalents in performKeyEquivalent:, before the key reaches any view, so a menu item holding ⌘W makes every view-level ⌘W binding unreachable. zed and Safari move it for the same reason.

Consequences worth flagging

• ⌘W no longer closes the window. ⌘⇧W does, and that is now the action's only binding — gpui derives the menu equivalent from the keymap, and a second ⌘W binding risks handing the menu back the chord it just gave up. Outside the terminal, ⌘W does nothing.
• On Windows and Linux, select-all and close-tab resolve to Ctrl+A and Ctrl+W, shadowing readline's beginning-of-line and delete-word inside the terminal. Same trade VS Code makes; shifted forms are the alternative if it grates.
• New-tab keeps literal Ctrl on macOS rather than Cmd, matching VS Code and zed.

Scoping select-all and close-tab to the Terminal context is also what gives them precedence: gpui ranks matches by context depth before registration order.

Testing

comet-ui is 380/380 green, clippy and fmt clean. New coverage: select-all reaching into scrollback and not panicking on an empty grid; the binding table parsing, scoping, and resolving mod- per platform.

Verified in the running app that ⌘⇧W closes the window. The remaining chords are covered by unit tests but have not been through a manual pass end to end.

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[neo773]

@wingleeio
Could I get a review on this when you get some time?

[wingleeio]

@neo773 I'll review this and try to get it in tomorrow