feat(ui): inline collapsible thinking trail

[lavindeep]

Surfaces the model's reasoning inline in the full-screen app as a faint, collapsible thinking trail (DESIGN §31.19). Additive and gated — a default session with no streamed thinking is byte-identical.

What it does

• stream_thinking now retains the thinking text (display-only — never recorded in history or fed back to the model) on a per-reasoning-phase _Trail.
• Collapsed by default: first 10 non-blank lines under a thinking · N reasoning header; once lines are hidden, footer … +N hidden lines · press t to expand. Expanded: full trail + press t to collapse. Caret ▸/▾ (ASCII >/v).
• t toggles the latest trail — the active one while the model runs, the most-recent after. Gated to an empty dock with no active approval; older trails freeze (no per-trail selection, by design). With no trail it falls through to ordinary self-insert (event.data * event.arg, repeat-count included) so a message starting with t still types.
• Any non-thinking transcript content finalizes the active trail, so a tool call between two reasoning phases yields two separate inline blocks.
• show_reasoning=False builds no trail; cancel/error/stale-turn cleanup clears only the active trail and never resets finished ones.
• Every displayed thinking line is _sanitize_line-stripped (model-controlled text).

Verification

Phase gate (green):

ruff check .          → All checks passed!
ruff format --check . → 140 files already formatted
mypy shellpilot --strict → Success: no issues found in 80 source files
pytest -q             → 1447 passed

17 new tests (red-first), covering collapsed/expanded/footer wording, two-phase trails, new-turn-defaults-collapsed, abort/stale cleanup preserving finished trails, show_reasoning=False, sanitization, the show_plan_progress finalize invariant, idle-toggle cache-bust, and all t-key edges (toggle / literal fall-through / numeric-arg repeat / dock-text / during-approval).

Live validation — nemotron-3-super:cloud, full-screen app (SHELLPILOT_UI=app):

❯ smallest positive integer divisible by every integer 1..10? ...
▸ thinking · 229 reasoning        ← collapsed trail, live reasoning count, thinking streamed inline
⏺ run_command(python3 -c ...)     ← model proposed a tool → trail finalized
  MEDIUM command   Approve? [y]es / [e]dit / [n]o   ← `t` correctly inert during approval
  ❯ n   ⎿ ✗ declined by user
▸ thinking · 92 reasoning         ← SECOND trail (new reasoning phase)
2520
✓ done · 45s · 321 reasoning · 762 total

• t → latest flips to ▾ thinking · 92 reasoning + press t to collapse; the older 229 trail stays ▸ (untouched).
• t again → back to ▸; dock stays empty (toggled, not typed).

Confirmed end-to-end: real cloud thinking → inline collapsible trail, two-phase blocks, latest-only toggle, finished-trail reopen, approval-mode gating, done line.

Targets ui-rework. Does not touch main.