#31384 base red: revert #27580 (transcript columns and block spacing broke gallery and webview tests)

Reverts #27580 (044769ae5dd6), which broke base webviews on the push at 5273be91:

• gallery health: 10 and 11 newly broken states on shards 0 and 3. Among them:
  • agent-pane.gutter-select#{agent-to-prompt,prompt-to-agent,prompt-to-tool,tool-to-prompt}-gap: "Gap drag selected no text"
  • agent-pane.transcript-spacing#{compact,empty-footer,relaxed}: "Footer-to-sender gap -2px; expected 10/14/18px"
  • agent-pane.composer-rest-gap#gap-56: "The rest gap is 98.0px, not 56px"
  • agent-pane.transcript#thought-row and #grid-rhythm-hover
• react-apps-check (3): agent-pane-tool-group-overlap, agent-pane-recap-card and composer-model-quiet fail.

Every broken state is the transcript spacing that #27580 changed, and many states fail at once, so reverting is faster than fixing forward. The only conflicts were the base autofix's reformat of #27580's own lines in model.ts, continuationSpacing.test.ts and transcript-rhythm.test.ts, so those files go back to how they were before #27580. Later commits to the same files (App.tsx, composerLocation.css, design-tokens-baseline.jsonl) are kept, and nothing still imports the files #27580 added (transcriptGrid.ts, TranscriptGridCards.gallery.*).

#27580 can come back as a fresh PR rebased on base, with its webview and gallery verdicts green at that head.

Agent session: agent session

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Summary by cubic

Reverts #27580 (transcript columns and block spacing), which broke 10-11 gallery states and the react-apps-check webview tests on the push — gutter-select gap drags, the transcript footer-to-sender gap, the composer rest gap, and the thought-row rhythm all regressed.

Every broken state comes from #27580's spacing changes, so reverting is faster than fixing forward. The files #27580 added (transcriptGrid.ts, TranscriptGridCards.gallery.*) are removed and nothing imports them. The base autofix's reformat of model.ts, continuationSpacing.test.ts, and transcript-rhythm.test.ts is reverted with it; later commits to App.tsx, composerLocation.css, and design-tokens-baseline.jsonl are kept. The tool-group scroll test now asserts its transcript overflows the pane so the wheel has somewhere to scroll, and the turn-stopped baseline row maps to turnSummary-583 to match the fixture. #27580 can return as a fresh PR rebased on base once its webview and gallery checks pass.