feat: rework the TUI transcript into collapsible episodes

[arelchan]

Summary

Reworks the TUI transcript so a turn reads as a sequence of steps instead of
interleaved thinking blocks and tool lines. Every model call becomes one
"episode" holding its reasoning, narration and tools.

Rendering

• A step with no narration collapses to one summary line ("reasoning for 8s,
  read 2 files"); a narrated or running step shows a reasoning fold, the
  narration as prose, then its tool rows.
• Reasoning expands while it streams and folds once the step produces anything
  visible (narration, a tool, or the answer text).
• A run of same-name calls renders as a tree: one header plus a child row per
  call, so a burst of parallel searches no longer fills the screen.
• In-flight tools report elapsed time; finished ones keep their measured span.
• Rows are bound to the transcript width and clipped with an ellipsis. Without
  the bound, long details overflowed the container and the terminal soft-wrapped
  them, which looked like stray blank lines and edge-cut text.

Tool display contract

Tools now own how they appear. Tool.display_call and the new ToolResult
(model_text plus optional display_text) separate what the model reads from
what the transcript shows:

• ask_user renders its question and the answer instead of a raw arguments
  blob, pairing each question with its answer when several are asked at once.
• Tools with no explicit verb derive a readable one from their own name, so a
  newly added tool renders sensibly with no changes to the verb table (it is an
  overrides list, not a required registry).
• The call preview picks the query, question or command out of the arguments
  instead of the first value, which could be a numeric flag.

Also

• Episodes are the default transcript style; /transcript legacy stays as an
  escape hatch and the legacy path is untouched.
• An interrupt commits the collapsed episode view instead of dumping the raw
  expanded segments.
• The virtual-height estimate accounts for episode rows. Without it the
  transcript reserved too few rows and left stale cells on screen.
• Markdown table columns fit the available width with CJK-aware clipping.
• The deep-research offer states that a regular search is the user's chosen
  path, not a failure, so the model stops narrating it as a broken fallback.

Type

• Fix
• Feature
• Docs
• CI / tooling
• Refactor
• Other

Verification

cd ui-tui && npm run type-check          # clean
cd ui-tui && npm test                    # 927 passed | 3 skipped
cd ui-tui && npm run lint                # 0 errors, no warnings in touched files
cd ui-tui && npx prettier --check "src/**/*.{ts,tsx}"   # clean
uv run ruff check raven/ tests/          # All checks passed
uv run ruff format --check raven/ tests/ # 758 files already formatted
uv run pytest tests/ -k "spine or tui_rpc or ask_user or deep_research or agent_loop" \
  --ignore=tests/integration             # 725 passed

New tests cover the episode commit path (including interrupt), the tool summary
and verb fallback, the argument preview, and the rendered layout.

• Relevant tests pass locally
• Relevant lint / type checks pass locally
• User-facing docs or screenshots are updated when needed

Risk

• Security impact considered
• Backward compatibility considered
• Rollback path is clear for risky changes

The wire gains one optional tool.start field (display); older payloads
without it fall back to the generic preview. execute may now return either a
string or a ToolResult, so existing tools are unaffected. Rollback is
/transcript legacy for the view, or reverting this commit.

Known follow-ups, not addressed here: occasional screen garbling seen when
messages are queued (root cause not yet identified), and web_search being
registered even without an API key.

Related Issues

N/A