feat: continue conversations across providers

[IchenDEV]

Summary

• switch an existing idle Session to another provider in place while preserving its id, transcript, policies, and workspace
• initialize and persist the replacement atomically, fence stale callbacks, roll back safely, and send only bounded provider-neutral continuation context
• retain the current unified provider/model picker, lock unsafe states, and synchronize focused panes, background panes, plugins, and the TUI
• add a real-provider canary that requires a non-empty durable transcript and continuity-key recall across every switch

Verification

• cargo test -p codetwo-core
• cargo test -p codetwo-core --test engine_provider_switch
• cargo check --workspace --all-targets
• cargo test -p codetwo-tui provider_change_reconciles_the_active_tui_shell
• bun test tests/sceneChip.test.tsx tests/canvasDesktop.test.ts
• bun run build:renderer
• live Codex -> Grok -> Cursor -> Codex continuity canary
• live OpenCode -> Kimi -> OpenCode continuity canary
• in-app Browser smoke check of the unified picker with a clean console
• documentation, SDLC, worktree, and diff Gates

Boundaries

Provider-native cursor, model, config, goal, tool, and MCP state are intentionally reset instead of migrated. Switching is rejected during active turns, user-input waits, concurrent switches, or managed Task leases. This PR does not authorize merge, release, or deployment.