refactor(robot): add constructors to RobotMeta, support optional gitlab_base_url

RobotMeta previously required direct struct literal construction with only
elapsed_ms. This made it impossible to add optional fields without updating
every call site to include them.

Introduce two constructors:
- RobotMeta::new(elapsed_ms) — standard meta with timing only
- RobotMeta::with_base_url(elapsed_ms, base_url) — meta enriched with the
  GitLab instance URL, enabling consumers to construct entity links without
  needing config access

The gitlab_base_url field uses #[serde(skip_serializing_if = "Option::is_none")]
so existing JSON envelopes are byte-identical — no breaking change for any
robot mode consumer.

All 22 call sites across handlers, count, cron, drift, embed, generate_docs,
ingest, list (mrs/notes), related, show, stats, sync_status, and who are
updated from struct literals to RobotMeta::new(). Three tests verify the
new constructors and trailing-slash normalization.

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

src/cli/commands/stats.rs

@@ -583,7 +583,7 @@ pub fn print_stats_json(result: &StatsResult, elapsed_ms: u64) {
                 }),
             }),
         },
-        meta: RobotMeta { elapsed_ms },
+        meta: RobotMeta::new(elapsed_ms),
     };
     match serde_json::to_string(&output) {
         Ok(json) => println!("{json}"),