feat(cli): expose available_statuses in robot mode and hide status_category

(Supersedes empty commit f3788eb — jj auto-snapshot race.)

Three related refinements to how work item status is presented:

1. available_statuses in meta (list.rs, main.rs):
   Robot-mode issue list responses now include meta.available_statuses —
   a sorted array of all distinct status_name values in the database.
   Agents can use this to validate --status filter values or display
   valid options without a separate query.

2. Hide status_category from JSON (list.rs, show.rs):
   status_category is a GitLab internal classification that duplicates
   the state field. Switched to skip_serializing so it never appears
   in JSON output while remaining available internally.

3. Simplify human-readable status display (show.rs):
   Removed the "(category)" parenthetical from the Status line.

4. robot-docs schema updates (main.rs):
   Documented --status filter semantics and meta.available_statuses.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/cli/commands/show.rs

@@ -628,13 +628,9 @@ pub fn print_show_issue(issue: &IssueDetail) {
     println!("State:    {}", state_styled);
 
     if let Some(status) = &issue.status_name {
-        let display = match &issue.status_category {
-            Some(cat) => format!("{status} ({})", cat.to_ascii_lowercase()),
-            None => status.clone(),
-        };
         println!(
             "Status:   {}",
-            style_with_hex(&display, issue.status_color.as_deref())
+            style_with_hex(status, issue.status_color.as_deref())
         );
     }
 
@@ -944,6 +940,7 @@ pub struct IssueDetailJson {
     pub closing_merge_requests: Vec<ClosingMrRefJson>,
     pub discussions: Vec<DiscussionDetailJson>,
     pub status_name: Option<String>,
+    #[serde(skip_serializing)]
     pub status_category: Option<String>,
     pub status_color: Option<String>,
     pub status_icon_name: Option<String>,