Fix transcript scrolling and live reasoning visibility during streaming

[SivanCola]

Closes #17.

The report is two separate problems in the chat webview.

1. The transcript could not be scrolled up while a turn was streaming.
Both auto-scroll checks were state.running || <near the bottom>, so as long as a turn was running every render and every transcript patch ran transcript.scrollTop = transcript.scrollHeight. Scrolling up during output was impossible — the next token pulled the view straight back down.

Auto-follow now keys off the reader instead of the turn: the transcript sticks to the bottom only when it is already there, and re-attaches by itself when the reader scrolls back down. The two places that still deserve a forced jump ask for one explicitly — sending a prompt, and returning from the settings view during a live turn (hiding the transcript destroys its scroll offset).

The geometry is now sampled before the DOM mutation. The old check in patchTranscript ran after the new nodes were appended, so once running was false it measured an already-grown transcript and skipped the follow it should have done.

2. Reasoning could not be read until the turn moved past it.
renderThought built a collapsed <details> showing only a one-line preview, and patchTranscript removes and re-creates every node from patch.start on. The streaming thought sits exactly at patch.start, so it was rebuilt collapsed on every delta and any manual expansion was thrown away a few milliseconds later.

Reasoning is now expanded by default in a height-capped (~15 line) internally scrolling body that follows the newest text, and the collapse state and scroll offset live outside the DOM so they survive the rebuild. Scrolling up inside a reasoning body parks it there; scrolling back to its bottom re-attaches the follow. Both are keyed by transcript index and dropped when the session changes.

The reasoning behaviour is adapted from @VaterVato's fork in #16 (01cf997, bf509a3), which fixed this half of the issue but carries a large set of unrelated changes; that work is credited as co-author on the commit. The transcript scroll lock is untouched on that branch and is fixed here.

Notes for review

• src/transcriptScroll.ts is a new module holding the pure scroll decisions so they are testable — webview.ts cannot be imported under node --test (it calls acquireVsCodeApi() at module scope).
• The two identical settings-close handlers were folded into closeSettings(), which is where the forced re-pin lives.
• toggle and inner scroll do not bubble, so both listeners are registered on the transcript in the capture phase.

Verification

• npm run lint, npm test (80 pass, 7 new in test/transcriptScroll.test.ts), npm run test:vscode (exit 0).
• Not verified by watching a live stream in a real Extension Host — the DOM wiring around the pure helpers is reviewed but untested.