Installer now prompts for OmniRoute base URL and apiMode, prefilled from
the existing opencode.json/opencode.jsonc. Defaults are localhost:20128/v1
and chat mode. Adds --base-url, --api-mode, and -y/--yes flags for CI/scripts.

JSONC support: detects opencode.jsonc/tui.jsonc, parses comments and
trailing commas, writes back to the same file. Warns once per file when
JSONC-only syntax will be lost on rewrite.