[FEATURE]: Tab to extend a question option into a custom answer

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

The closest existing issues (#30028, #34476) target different mechanisms — see "Relationship to existing issues" below.

Describe the enhancement you want to request

Problem

The question prompt lets you pick an option verbatim, or start a blank "Type your own answer". But very often the right answer is "basically option 1, but…". Today that means ignoring the presented option and retyping the whole thing in the custom field.

Proposed solution

Let people extend a highlighted option in place. Highlight an option, press Tab, and it opens for inline editing seeded with "<label>, ", cursor at the end, so you just add the qualifier.

Do you want to use Three.js to create your app?
> 1. Yes            ── Tab ──▶   1. Yes, ▌   ◀ type the rest
  2. No                          2. No
  3. Chat about it               3. Chat about it

Enter commits the extended answer (e.g. Yes, but only for certain parts of it); Esc cancels.

Behavior (all open to change)

• Tab on a concrete option → edit in place, prefilled with "<label>, ".
• Tab still switches questions when on the "Type your own answer" row or the Confirm tab (multi-question prompts).
• Only enabled when the question allows custom answers (custom !== false).
• Reuses the existing custom-answer submit path; result is delivered as a custom answer string.
• Mouse hover no longer moves the selection while editing, so an in-progress edit can't be relocated.

Relationship to existing issues

• #30028 (input field on options): model-driven — the LLM marks a specific option as a fill-in-the-blank, and it needs a schema change. This proposal is user-driven and needs no schema change: the user can extend any option on demand. The two are complementary; this could ship as the pure-UX half of that idea.
• #34476 (auto-enter edit mode on the "Type your own answer" row): about the generic custom row only. This proposal is about extending a specific presented option, anchored to its text.

Status

Working implementation, single file (packages/tui/src/routes/session/question.tsx), passing bun typecheck (whole workspace) and oxlint. Happy to open a PR once the interaction is blessed.

Per CONTRIBUTING.md, raising this for design review before implementation. Preferences welcome on: the key binding (Tab vs. other), submit semantics, and behavior for multi-select questions.