feat(explain): implement lore explain command for auto-generating issue/MR narratives

Adds the full explain command with 7 output sections: entity summary, description,
key decisions (heuristic event-note correlation), activity summary, open threads,
related entities (closing MRs, cross-references), and timeline excerpt (reuses
existing pipeline). Supports --sections filtering, --since time scoping,
--no-timeline, --max-decisions, and robot mode JSON output.

Closes: bd-2i3z, bd-a3j8, bd-wb0b, bd-3q5e, bd-nj7f, bd-9lbr

src/cli/mod.rs

@@ -277,6 +277,44 @@ pub enum Commands {
     /// Trace why code was introduced: file -> MR -> issue -> discussion
     Trace(TraceArgs),
 
+    /// Auto-generate a structured narrative of an issue or MR
+    #[command(after_help = "\x1b[1mExamples:\x1b[0m
+  lore explain issues 42                  # Narrative for issue #42
+  lore explain mrs 99 -p group/repo       # Narrative for MR !99 in specific project
+  lore -J explain issues 42               # JSON output for automation
+  lore explain issues 42 --sections key_decisions,open_threads  # Specific sections only
+  lore explain issues 42 --since 30d      # Narrative scoped to last 30 days
+  lore explain issues 42 --no-timeline    # Skip timeline (faster)")]
+    Explain {
+        /// Entity type: "issues" or "mrs" (singular forms also accepted)
+        #[arg(value_parser = ["issues", "mrs", "issue", "mr"])]
+        entity_type: String,
+
+        /// Entity IID
+        iid: i64,
+
+        /// Scope to project (fuzzy match)
+        #[arg(short, long)]
+        project: Option<String>,
+
+        /// Select specific sections (comma-separated)
+        /// Valid: entity, description, key_decisions, activity, open_threads, related, timeline
+        #[arg(long, value_delimiter = ',', help_heading = "Output")]
+        sections: Option<Vec<String>>,
+
+        /// Skip timeline excerpt (faster execution)
+        #[arg(long, help_heading = "Output")]
+        no_timeline: bool,
+
+        /// Maximum key decisions to include
+        #[arg(long, default_value = "10", help_heading = "Output")]
+        max_decisions: usize,
+
+        /// Time scope for events/notes (e.g. 7d, 2w, 1m, or YYYY-MM-DD)
+        #[arg(long, help_heading = "Filters")]
+        since: Option<String>,
+    },
+
     /// Detect discussion divergence from original intent
     #[command(after_help = "\x1b[1mExamples:\x1b[0m
   lore drift issues 42                    # Check drift on issue #42