Accept any capitalization at init's environment prompt

[jbedient-kizen]

kizen init's Environment prompt rejected correctly-typed answers because of their case, and never revealed its own escape hatch.

What was wrong

Rich's choices matching is case-sensitive by default. Typing Go for a go business — the natural way to capitalize it — looped forever on "please select one of the available options" with no indication why. From the user's side that is indistinguishable from the prompt not accepting input at all.

Separately, Rich's stock rejection message never mentioned that url is itself one of the listed choices and opens a second free-text prompt. Anyone whose environment is not one of the curated names had no way to discover that, and could end up re-typing their real host indefinitely.

What changed

• _ask passes case_sensitive=False, so capitalization no longer matters.
• A small _EnvironmentPrompt subclass overrides illegal_choice_message to say directly: Type "url" to enter a custom address instead.

Both are confined to cli/init.py. No flags, no --help changes, so scripts/cli-tree-baseline.txt is untouched.

Verification

bin/check.sh builder-cli — lint, format, typecheck, test, extra_checks all pass: 1251 passed, 5 skipped, 76 deselected. (The 5th skip is the chdb importorskip, legitimate without --extra connectors.)

The two new tests were confirmed to genuinely cover the fix rather than pass either way — with init.py reverted to main they both fail:

FAILED tests/test_cli.py::test_init_environment_picker_is_case_insensitive
FAILED tests/test_cli.py::test_init_environment_picker_mistyped_name_shows_url_hint
2 failed, 98 deselected