Open new links in same window, not new window

Currently, goose://recipe?config=... deep links always open a new Goose window. For investigators and power users who trigger recipes frequently from internal tools (like G2 apps or Block cell sites), this creates window clutter and breaks their workflow. We'd love the option to open a recipe deep link as a new chat tab in the existing Goose window rather than spawning a separate window each time. A possible implementation could be a URL parameter like goose://recipe?config=...&target=current that signals Goose to reuse the focused window and start a new session/tab within it. This would keep the investigator's workspace clean, reduce context switching, and align with how most tabbed applications handle "open in new tab" behavior. Several teams at Block (SIU, PayX, CS EU/UK, Finance Strategy) are building tools that launch Goose recipes from web apps, and all of them would benefit from this improvement.

- [x] I have verified this does not duplicate an existing feature request

Thank you team!!

Look at the deep link handling code, likely in a module like `src/deep_link.rs` or `src/window_manager.rs`. Find where `goose://recipe` URLs are parsed and new windows are spawned. Add support for a `target=current` URL parameter to reuse the focused window and create a new chat tab. Test by launching a recipe deep link with the parameter from an external tool.

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