Migrate CLI argument parsing to betteropts

[jakub-bochenski]

Adapts the bin/sandcat dispatcher (module/command listing, --__complete, --completion), adds betteropts as a submodule, and converts every libexec command to declare its flags/options/arguments through it instead of hand-rolled while/case loops. Bumps the minimum Bash version to 4.2 since betteropts requires associative arrays and declare -g.

Behavioral notes:

• init's --agent/--ide/--secret-provider (and its --sp alias) validate via betteropts' own type=choice/choices=, since each is a single value from a small fixed (or, for --agent, dynamically fetched) set - a genuine fit. --stacks/--features stay on custom validation since they're comma- separated multi-value fields, not a single choice; cache/rm and init/devcontainer have no choice-shaped validation to convert either.
• init's --stacks/--secret-provider(/--sp)/--features need to distinguish "explicitly passed empty" from "omitted" (to skip interactive prompts), which betteropts doesn't expose directly; a small presence-scan over the raw args recovers it alongside the normal betteropts value parsing. The same reasoning shows up in test/init/init.bats: betteropts schema state is process-global, so tests that call init twice route both calls through run (which forks a subshell) to keep the first call's schema from leaking into the second.

[adamw]

Thanks - though I'll hold off with merging this since it requires a bash update on mac, which would increase the barrier of entry for some users. But if people have other opinion, let me know :)

[jakub-bochenski]

FWIW the shell installer (#80) already has to handle jq, so if it's handling one dependency IMO adding another one is not such a big jump.