Display GitHub stars in directory search/list results

[JTP75]

Description

vettd search, vettd list, and vettd view render directory results from the vettd directory API via the allow-list structs in crates/vettd-cli/src/directory.rs (DirectoryCard, DirectorySkillDetail), but neither struct nor the rendered output currently has a field for GitHub star count. Once the vettd directory API exposes star counts for GitHub-imported skills (AgenticHighway/vettd#782), the CLI should surface that number so users can gauge popularity when browsing search/list results.

Scope

• Consume the star count field from the directory API's list/search/detail responses
• Display star count in the table output for search/list and in the view detail output
• Include star count in --json output mode

Acceptance Criteria

• search/list table output shows GitHub star count when available
• view output shows GitHub star count when available
• --json output includes the star count field
• Missing/null star count (e.g. non-GitHub sources) renders gracefully

[JTP75]

Solution direction (depends on AgenticHighway/vettd#782 first exposing a star count on the directory API skill card).

The directory response types in crates/vettd-cli/src/directory.rs are forward-compatible allow-list structs (unknown server fields are ignored). Add an optional star field (Option<u32>) to DirectoryCard and DirectorySkillDetail.

Render the star count in the search/list table output (print_cards / print_card_row) and in the view detail output (handle_view) when present. Because --json serializes these structs directly, adding the field also covers JSON mode. Skills without a star count (non-GitHub sources, or an older server that omits the field) must render gracefully — a neutral placeholder, not 0.

Leave exact column/formatting choices to the implementer.

Optional follow-up (not required for this issue): a stars value for --sort (currently newest|rating|alpha) mapping to the server's sort=stars. Scope this issue to display; sorting can be a separate change.