chore(mcp): convert lib/local-branch.js to TypeScript (Phase 2 of #7291)

Convert packages/loopover-mcp/lib/local-branch.js to real TypeScript,
using the tsc build pipeline landed in Phase 1 (#7409). Types only;
runtime behavior is byte-identical (the compiled lib/local-branch.js
changes only in tsc formatting and the elision of an already-dead
node:path `join` import).

Remove the now-unused `@ts-expect-error package helper is plain JS`
directives above the dynamic local-branch.js imports in
test/unit/local-branch.test.ts and test/unit/local-scorer-adapter.test.ts,
which now resolve to the typed .ts source.

Closes #7329

Author: jaytbarimbao-collab