refactor: unwrap PluginLoader namespace + self-reexport (redo)

[kitlangton]

Summary

Redo of #22946 — the original PR merged cleanly with all checks green but the commit was later dropped from `origin/dev` by a release-pipeline force-push. Same content as the dropped `75cbba355`, cherry-picked onto current `dev`.

Unwrap `export namespace PluginLoader` in `packages/opencode/src/plugin/loader.ts` into flat top-level exports + self-reexport for tree-shaking and Node native TS runner compatibility. Public API preserved via appended `export * as PluginLoader from "./loader"`. No consumer imports change.

Why this needs a redo

A release-pipeline force-push rewrote `dev` after the original merge, dropping multiple of our namespace-unwrap commits. Detected during a clean-sweep audit of remaining `export namespace` declarations in `packages/opencode/src/`. Plain cherry-pick of the original commit applies cleanly (the file was not touched on dev since the drop).

Verification

• `bunx --bun tsgo --noEmit` from `packages/opencode/` — 0 errors
• `bun run --conditions=browser ./src/index.ts generate` — no SDK drift
• `test/plugin/` — 90/90 pass across 11 files

Diff

packages/opencode/src/plugin/loader.ts | 278 ++++++++++++-------
1 file changed, 139 insertions(+), 139 deletions(-)

Symmetric de-indent, same shape as every other tier-1 unwrap PR.