Single source of truth for 'is this a known CLI command' #1043

Context

Command-knowledge now lives in two places: the catalog-derived early check in src/cli/parser/args.ts (isCommandKnown, added by #1038 with a sync-warning comment) and the authoritative dispatch fall-through in cli.ts (~line 408) that throws Unknown command after all runners decline. The duplication was created knowingly to fix error ordering (#1036); today the catalog covers all special-cased commands (verified: auth/connection/mcp/proxy/web are in LOCAL_CLI_COMMANDS), but any future command wired only into cli.ts will be rejected at parse time and the failure will look like a user typo.

Expectation

One exported predicate (catalog module is the natural home) consumed by both the parser early-check and anything else that needs command existence; the cli.ts fall-through remains as a defensive assert, ideally referencing the predicate. Acceptance: isCommandKnown in args.ts deleted or reduced to the shared call; a test enumerating every command cli.ts can dispatch against the predicate so drift fails loudly.