feat(chat): add provider thread history actions

[astyfx]

Summary

Adds provider-native conversation history actions and a Codex/BEUI-inspired turn preview rail, with Claude/Codex capability parity and explicit explanations for unsupported actions.

Changes

Features

• chat: add an accessible turn preview rail with jump, fork, and rollback actions
• providers: add Claude/Codex point-in-time fork, Codex rollback, and native rename synchronization
• docs: document conversation history actions and publish the feature guide

Fixes

• persistence: preserve unloaded browser-fallback task message pages during workspace snapshots
• chat: isolate rail navigation from conversation scroll restoration

Test Evidence

• bun run test:isolated — 334 test files passed after merging current main
• bun run typecheck
• bun run build
• bun run build:desktop
• bun run build:pages
• bun run check:reliability-gates
• bun run check:max-lines-ratchet
• bun run check:switch-exhaustiveness
• PLAYWRIGHT_WEB_PORT=4194 bun run test:e2e -- tests/e2e/chat-scroll.e2e.ts --project=chromium --workers=1 — 6 passed
• Lens QA: dark/light surfaces, 48×24 px targets, provider capability states, keyboard behavior, and reduced-motion behavior

Notes

• Workspace files are intentionally unchanged by fork or rollback.
• Claude does not expose native in-place rollback; the disabled action remains visible with an explanatory tooltip.

[heath-s]

Approved