feat(timeline): rename --expand-mentions to --no-mentions, default mentions on

Invert the timeline mention-expansion flag semantics. Previously, mention
edges were excluded by default and --expand-mentions opted in. Now mention
edges are included by default (matching the more common use case) and
--no-mentions opts out to reduce fan-out when needed.

This is a breaking CLI change but aligns with the principle that the
default behavior should produce the most useful output. Users who were
passing --expand-mentions get the same behavior without any flag. Users
who want reduced output can pass --no-mentions.

Updated: CLI args (TimelineArgs), autocorrect flag list, robot-docs
schema, README documentation and flag reference table.

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

src/cli/autocorrect.rs

@@ -166,7 +166,7 @@ const COMMAND_FLAGS: &[(&str, &[&str])] = &[
             "--project",
             "--since",
             "--depth",
-            "--expand-mentions",
+            "--no-mentions",
             "--limit",
             "--fields",
             "--max-seeds",