Tracking B3a: per-step change cards and per-file/per-step rollback #1401

Tracking issue for the B3a PR (branch feat/change-cards-rollback-b3a; PR to be opened after review).

Part of the file-write safety series — plan: easonLiangWorldedtech/Zoo-Code#33, epic: #1375. Item: B3a — Per-step change cards + per-file/per-step rollback (extension host). Stacks on the B1 per-write checkpoint PR (#1397) and the B2 change journal PR (#1398); the webview UI for the cards ships separately as B3b.

Scope

• Change card payload per assistant step (the currentStreamingDidCheckpoint span): summary (filesChanged/added/removed) + per-file entries; the unified diff per file is included only when the new changeCardDetail setting is "full" (default "summary"). The card rides the existing checkpoint_saved say as an additive optional field — no new say/webview types.
• Per-file rollback: content-only restore of one file from a checkpoint via the shadow git blobs (git show <hash>:<path> + S3 safeWriteText); created-after → delete; never truncates the checkpoint list, never rewinds messages, never cancels the task.
• Per-step rollback: uses the B2 journal as the step index to restore every file the step touched, one by one.
• Two new additive webview message cases (checkpointRestoreFile / checkpointRestoreStep) with zod payloads and the existing error-surface pattern; i18n error keys in every locale.
• changeCardDetail persisted setting, full round trip (packages/types, ClineProvider ×2, SettingsView cachedState payload, i18n label).

Acceptance

• One card per step (multiple writes aggregate); auto-approved writes get the same card
• File/step rollback restores the step's own content, touches no other file, and leaves conversation state + task lifecycle untouched (proven by test)
• changeCardDetail round-trips stored/garbage/unset correctly ("summary" default)
• CI green (ubuntu unit-test gate), 100% codecov patch coverage

Status

• PR: to be opened (after B1+B2 review/ship)