fix(cli): add zig to the --language choices for parse and scan

[gadievron]

fix(cli): add zig to the --language choices for parse and scan

Problem

openant parse --language zig (and scan) fails with argument --language/-l: invalid choice: 'zig'. The Zig parser is fully wired everywhere else: core/parser_adapter.parse_repository dispatches language == "zig", detect_language() returns "zig" and lists it in its docstring + the "no source files" error, and config/languages.json maps .zig → zig. Only the explicit --language argparse choices list in openant/cli.py (for both parse and scan) was never updated — so Zig was reachable via --language auto but not by explicit selection.

Fix

Add "zig" to the choices list at both cli.py sites (parse + scan). Purely additive; exposes nothing new — auto already routed .zig repos to the Zig parser.

Verification

$ openant parse /tmp/zig-kf --language zig --level reachable
  Found 2 Zig files
  After reachable filter: 9 functions     # was: error: invalid choice: 'zig'

Not a security fix

CLI usability fix.

Tests

No test pins the --language choices list (verified: grep -rn 'choices=\[.auto' tests/ = 0). The change is a 2-line argparse addition.

Depends-on

None (standalone; the choices list is pre-existing on master).