agent: Improve discoverability of the quote selection action (#40897)

This PR renames the `agent::QuoteSelection` to
`agent::AddSelectionToThread` _and_ adds it as a menu item in both the
right-click context menu within regular buffers as well as the
"Selection" app menu.

We've received feedback in the past about how hard to discover this
feature is, and after watching [the Syntax podcast
crew](https://www.youtube.com/watch?v=bRK3PeVFfVE) recently struggle
with doing so—and then naturally looking for it in the context menu and
not finding it—it felt like time to push a change. I think the rename +
the availability in these places could help bringing it to surface more.

The same action can be done in Cursor through the `cmd-l` keybinding,
but in Zed, that triggers `editor::SelectLine`, which I don't want to
override by default. However, if you're using Cursor's keymap, then
`cmd-l` does trigger this action, as expected.

<img width="500" height="1812" alt="Screenshot 2025-10-22 at 12  01@2x"
src="https://github.com/user-attachments/assets/dfc2c41c-8d0a-4a1a-8ea1-1bd5d1aa1171"
/>


Release Notes:

- agent: Improves discoverability of the previously called "quote
selection" action—which allows to add a text selection in a buffer as
context within the agent panel—by renaming it to "add selection to
thread" and making it available from the right-click editor context menu
as well as the "Selection" app menu.

assets/keymaps/default-linux.json

@@ -139,7 +139,7 @@
       "find": "buffer_search::Deploy",
       "ctrl-f": "buffer_search::Deploy",
       "ctrl-h": "buffer_search::DeployReplace",
-      "ctrl->": "agent::QuoteSelection",
+      "ctrl->": "agent::AddSelectionToThread",
       "ctrl-<": "assistant::InsertIntoEditor",
       "ctrl-alt-e": "editor::SelectEnclosingSymbol",
       "ctrl-shift-backspace": "editor::GoToPreviousChange",
@@ -243,7 +243,7 @@
       "ctrl-shift-i": "agent::ToggleOptionsMenu",
       "ctrl-alt-shift-n": "agent::ToggleNewThreadMenu",
       "shift-alt-escape": "agent::ExpandMessageEditor",
-      "ctrl->": "agent::QuoteSelection",
+      "ctrl->": "agent::AddSelectionToThread",
       "ctrl-alt-e": "agent::RemoveAllContext",
       "ctrl-shift-e": "project_panel::ToggleFocus",
       "ctrl-shift-enter": "agent::ContinueThread",