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/autocorrect.rs

@@ -209,6 +209,16 @@ const COMMAND_FLAGS: &[(&str, &[&str])] = &[
         ],
     ),
     ("drift", &["--threshold", "--project"]),
+    (
+        "explain",
+        &[
+            "--project",
+            "--sections",
+            "--no-timeline",
+            "--max-decisions",
+            "--since",
+        ],
+    ),
     (
         "notes",
         &[
@@ -388,6 +398,7 @@ const CANONICAL_SUBCOMMANDS: &[&str] = &[
     "file-history",
     "trace",
     "drift",
+    "explain",
     "related",
     "cron",
     "token",