feat: revamp elicitation forms for free-text input

[william0wang]

Summary

Reworks the elicitation form rendering so users can enter custom free-text
answers alongside the model-suggested dropdown, instead of being restricted
to enum-only options.

Problem

ACP/MCP elicitation string fields are EITHER an enum (restricted dropdown) OR
free text — the spec forbids a single field that is both. The previous
rendering used a plain enum for each question, so users could only pick
from the model's suggestions with no way to type their own answer.

Changes

AskUserQuestion — dual-field rendering

Each question becomes two fields:

• q_<i>: enum dropdown (now oneOf/anyOf with {const, title}) + a
  trailing "Skip this question" option (sentinel __skip__).
• q_<i>_other: free-text companion. Non-empty value overrides the dropdown
  (single-select) or is appended to the picks (multi-select, deduped).

No field is required; blank = skip just that question.

ExitPlanMode — single feedback field

Replaced approve/reject enum with one feedback text field:

• submit empty → approve
• submit with text → reject, text becomes the decline reason
• cancel button → plain reject

Docs & tests

• docs/PROTOCOL.md updated with the new form shapes.
• tests/interaction.test.ts expanded (free-text override,