feat(me): add mentions section for @-mentions in dashboard

Add a new --mentions flag to the `lore me` command that surfaces items
where the user is @-mentioned but NOT already assigned, authoring, or
reviewing. This fills an important gap in the personal work dashboard:
cross-team requests and callouts that don't show up in the standard
issue/MR sections.

Implementation details:
- query_mentioned_in() scans notes for @username patterns, then filters
  out entities where the user is already an assignee, author, or reviewer
- MentionedInItem type captures entity_type (issue/mr), iid, title, state,
  project path, attention state, and updated timestamp
- Attention state computation marks items as needs_attention when there's
  recent activity from others
- Recency cutoff (7 days) prevents surfacing stale mentions
- Both human and robot renderers include the new section

The robot mode schema adds mentioned_in array with me_mentions field
preset for token-efficient output.

Test coverage:
- mentioned_in_finds_mention_on_unassigned_issue: basic case
- mentioned_in_excludes_assigned_issue: no duplicate surfacing
- mentioned_in_excludes_author_on_mr: author already sees in authored MRs
- mentioned_in_excludes_reviewer_on_mr: reviewer already sees in reviewing
- mentioned_in_uses_recency_cutoff: old mentions filtered
- mentioned_in_respects_project_filter: scoping works

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/cli/mod.rs

@@ -1103,6 +1103,10 @@ pub struct MeArgs {
     #[arg(long, help_heading = "Sections")]
     pub activity: bool,
 
+    /// Show items you're @mentioned in (not assigned/authored/reviewing)
+    #[arg(long, help_heading = "Sections")]
+    pub mentions: bool,
+
     /// Activity window (e.g. 7d, 2w, 30d). Default: 30d. Only affects activity section.
     #[arg(long, help_heading = "Filters")]
     pub since: Option<String>,
@@ -1131,7 +1135,7 @@ pub struct MeArgs {
 impl MeArgs {
     /// Returns true if no section flags were passed (show all sections).
     pub fn show_all_sections(&self) -> bool {
-        !self.issues && !self.mrs && !self.activity
+        !self.issues && !self.mrs && !self.activity && !self.mentions
     }
 }