Show proposals in the code: ghost cursor, rationale tooltip, accept/reject CodeLens

[Seungpyo1007]

Closes track B of the 0.3.0 plan — the three items the survey settled on.

Until now a proposal lived only on the card to the right. Seeing what was about to change meant looking away from the code, and deciding meant clicking over there too. The design put all of it on the changed line.

Ghost cursor

editAnimator now draws a caret at the exact character being typed, as a zero-length decoration's pseudo-element — zero width, so it does not push the text. A whole-line highlight cannot tell you where to look on a long line. It does not blink for anyone who asked for reduced motion.

Rationale tooltip and accept/reject CodeLens

Both answer the same question: where does this proposal attach in the text as it stands right now. That answer lives in src/review/proposalMarks.ts as pure functions; src/editor/proposalDeco.ts puts it on screen.

The part that matters: the resolution order matches applyProposal exactly. If it did not, the range shown on the card and the range drawn in the editor could differ — and that means someone reads one place and clicks accept on another. When the snippet cannot be found, or occurs twice, locate returns null and nothing is drawn. Drawing in the wrong place is worse than not drawing.

Documentation

Phase 2 was written as "fork Code-OSS". That fork never happened and should not: forking existed to reach the renderer, and this app has had its own renderer from the start. The item closes as not-applicable and Phase 2 is marked done against the four pillars instead. The provider list was also stale — Gemini and local servers were missing.

Verification

A stub model emitting a single propose_edit was attached through the local provider, so a real proposal went through the real agent loop — the local-model work from #72 became the test rig.

7/7: the proposal arrives; the changed line is marked; hovering shows local — b 가 1부터 세는 곳에서 쓰여 22 로 올린다; the lenses render as ✓ 수락 | ✕ 거절; the ghost caret is present while the edit types in; the edit lands; the mark clears afterwards.

1063 unit tests passing (18 new on locate, including a stale range, a snippet that occurs twice, and a range pointing outside the file). npm run typecheck and npm run build clean.

One probe mistake worth noting: the hover popup covers the CodeLens, so the accept click was landing on the popup and the run reported that accepting did nothing. Moving the pointer away first fixed the probe; the app was fine.