test: fix 6 pre-existing typecheck errors so `npm run typecheck` is clean (#64)
The suite passes at runtime (vitest strips types), but `npm run typecheck`
(tsc over src+tests) was red with 6 errors that CI never caught — nix.yml
only runs `nix build` (src-only), not typecheck or vitest.
- tests/tui-framework.test.ts (x4): ScreenContext render mocks predated the
focus-manager work and were missing the now-required quit/focus fields.
Completed each mock (quit no-op + createFocusManager()).
- tests/exec.test.ts, tests/nesting.test.ts: `{ ...process.env, PTY_SESSION_DIR }`
loses the index signature, so `delete env.PTY_SESSION` didn't typecheck.
Annotated env as Record<string, string | undefined> (same pattern the other
test helpers already use).
Verified: `npm run typecheck` clean (0 errors); the 3 files pass at runtime
(95 tests).