Add back-navigation support to all interactive renderers

[ericof]

Summary

• Users can type < at any prompt to re-answer the previous question
• A hint (type < to go back) is shown from the second question onward; no hint on the first question
• Conditional questions respond correctly when the user goes back past a gating answer — the pipeline recomputes active questions on every iteration

Implementation

• Form.unrecord(key) — removes an answer from form.answers and form._user_answers
• Form.set_question_index(n) — sets the question index directly (used by the iterative loop)
• _GoBackRequest(Exception) — sentinel raised by _ask_* methods; caught by _ask_questions
• BaseRenderer._BACK_COMMAND = "<" — the back command string (override to change it)
• BaseRenderer._back_hint() — returns a hint string when going back is available, empty string otherwise
• BaseRenderer._iter_active_leaves() — recomputes active visible leaf questions dynamically each iteration
• BaseRenderer._ask_questions() — refactored from recursive to iterative with a history: list[str] stack

All three interactive renderers (StdlibRenderer, RichRenderer, CookiecutterRenderer) implement the pattern. NoInputRenderer is unaffected.

Documentation

• docs/src/reference/base-renderer.md — new "Back navigation" section documenting _GoBackRequest, _BACK_COMMAND, _back_hint(), and the implementation contract
• docs/src/how-to-guides/create-renderer.md — example renderer updated to support back navigation; new "Support back navigation" section added
• docs/src/tutorials/your-first-form.md — user-facing mention of the < command

Test plan

• test_back_on_second_question_rerenders_first — basic back navigation
• test_back_on_first_question_stays_at_first — no-op on first question
• test_back_multiple_times — multiple back steps
• test_back_from_boolean/choice/multiple_rerenders_previous_string — back across question types
• test_back_past_gating_question_hides_conditional — conditional question hidden after back + re-select
• test_back_past_gating_question_shows_conditional_on_reselect — conditional question shown after back + re-select
• test_back_hint_not_shown_for_first_question / test_back_hint_shown_for_second_question

Closes #10