feat(v2): add TUI plugin entry with /preset switcher

Task 5 of the phase-2 plan: new v2 TUI plugin entry (./tui export ->
dist/tui2.js) providing /preset on v2 hosts via ui.dialog.select +
ui.toast + keymap.layer, persisted through the shared switchPresetOnDisk
helper (dialog pick or /preset <name>; applies on next reload).

Composition instead of a v2-only module: ./tui already shipped the
dual-contract plugin (dist/tui.js, #1002), so src/v2/tui.ts re-exports
the identical v1 tui factory and wraps the existing v2 setup (sidebar)
with the preset keymap layer - v1 loading is unchanged, v2 gains both
surfaces in one entry. dist/tui.js is still built (release artifact
checks require it); the release smoke contract (id 'oh-my-opencode-slim:
tui', tui fn, setup fn) holds for the new entry.

v1 loader tolerance for the added `tui: true` field on the default
export: the v1.18.13 clone was unavailable locally, but
docs/opencode-v2-compatibility.md (phase-1 research on that same clone)
records that readV1Plugin only probes for a `server` field and ignores
extra keys; the shipped dual-contract precedent (#1002) confirms both
hosts ignore unknown fields. `tui: true` is inert on v1.

Also closes the Task 2 review gap: MultiplexerSessionManager is created
unconditionally and self-gates only on env, so on a v2 host inside tmux
it could self-enable. New sessionManagerMultiplexerConfig() forces type
'none' for hostFlavor === 'v2' (all manager methods no-op when
disabled); v1 hosts receive the same config object untouched.

build:tui externals mirror build:plugin (opentui/solid/plugin/sdk) with
zod inlined like dist/server.js; types point at dist/v2/tui.d.ts (tsc
mirrors the src tree).