fix: drop `tui: true` marker that rejects the plugin on v1.18.23+ hosts

The v2-compat work (PR #1109) added a boolean `tui` marker to the
default export based on an unverified assumption that v1 hosts ignore
extra keys. OpenCode v1.18.23+ — and v2's byte-identical readV1Plugin —
validate that a server plugin module's `tui` field is a function and
never coexists with `server`, so the marker made every such host reject
the whole plugin with "invalid tui export".

No host reads the marker: v2 discovers the TUI entry through the
package.json `./tui` export (dist/tui2.js), and the missing/retry
classification runs entirely pre-import. Remove it, document the loader
contract inline, and add a regression test pinning the validation.