Make CLI value flags strict and testable

[amazon7737]

Context

The CLI package owns argv parsing and side effects but currently has no test files. Some value flags can be omitted or followed by another flag without producing a clear usage error, which can silently fall back or skip configuration.

Acceptance criteria

• Reject missing values for value-taking flags such as --store, --viewer, and --port with actionable English usage errors.
• Reject unknown flags for the affected commands instead of silently ignoring them.
• Validate arguments before filesystem, network, process, or server side effects begin.
• Introduce the smallest test seam needed to exercise parsing and output with synthetic temporary roots and injected I/O or dependencies.
• Cover representative init, serve, and pr-comment argument cases without publishing comments or opening network listeners.
• Preserve documented valid command behavior.
• Pass targeted race tests and the repository-wide validation suite.

First safe step

Extract or wrap pure argument validation and add table-driven tests for missing values and unknown flags.

Non-goals

• Replacing the CLI framework wholesale.
• Publishing PR comments, using credentials, or changing production configuration.