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.

docs/src/ai/text-threads.md

@@ -16,7 +16,7 @@ To begin, type a message in a `You` block.
 
 As you type, the remaining tokens count for the selected model is updated.
 
-Inserting text from an editor is as simple as highlighting the text and running `agent: quote selection` ({#kb agent::QuoteSelection}); Zed will wrap it in a fenced code block if it is code.
+Inserting text from an editor is as simple as highlighting the text and running `agent: add selection to thread` ({#kb agent::AddSelectionToThread}); Zed will wrap it in a fenced code block if it is code.
 
 ![Quoting a selection](https://zed.dev/img/assistant/quoting-a-selection.png)
 
@@ -148,7 +148,7 @@ Usage: `/terminal [<number>]`
 
 The `/selection` command inserts the selected text in the editor into the context. This is useful for referencing specific parts of your code.
 
-This is equivalent to the `agent: quote selection` command ({#kb agent::QuoteSelection}).
+This is equivalent to the `agent: add selection to thread` command ({#kb agent::AddSelectionToThread}).
 
 Usage: `/selection`