refactor: consolidate installer defaults and FILESYSTEM validation into config.sh

The installer had three sites touching FILESYSTEM: a top-level default in the monolith, a re-default block in gather_input, and a runtime validation block also in gather_input. The same scattering existed for LOCALE, KEYMAP, ENABLE_SSH, and NO_ENCRYPT. A future contributor changing one site wouldn't have known the other two existed.

Move all five defaults into the lib/config.sh declarations so config.sh is the single source of truth. Add validate_filesystem() in lib/config.sh and call it from main() between check_config and gather_input, so a typo in a config file's FILESYSTEM= fails fast before any install action runs.

The behavior change is stricter. An empty FILESYSTEM in a config file used to be silently defaulted to zfs, now it errors. Interactive mode is unaffected. select_filesystem still controls the value and already errored on cancellation.

Bats: 140 → 142. Five tests added in test_config.bats for the defaults pinning and validate_filesystem coverage. Three removed from test_archangel.bats for behavior that moved out of gather_input. Lint clean.

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