search_add_to_chat

From each search_pane hit, a split **Add to chat** control:
- Primary button: place compact `#requirement` context into the **current** chat input without submitting.
- ▾ menu ( visible secondary affordance ): **Add to current chat** or **Add to new chat** ( new session, then seed the same payload ).
Tooltips name both behaviours; secondary is not right-click-only.
Same payload shape as add_to_chat ( idea name, path: line, summary ) via open-chat.ts.
Host: open-chat.ts with `{ query, isPartialQuery: true, target: 'current' | 'new' }`.
Cursor note: do not use `workbench.action.chat.open` for **current** ( it always opens a new tab ); see add_to_chat / open-chat.ts.
Clipboard fallback when no chat command is available ( same spirit as add_to_chat ).
Wire: AddToChatButton.svelte / SearchPane.svelte → app.svelte.ts `addToChat(hit, target)` → activity-bar-messages.ts → activity-bar-webview-provider.ts → open-chat.ts.

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