Sprint C: author-filtered stats scan, release cadence + repo size, weekly digest

- src/stats/: OwnEmailMatcher (config emails + GH no-reply per account) +
  git-side --author filtering (cheap even on the pterodactyl fork's 4700+
  commits); streak (current/longest, midnight-grace rule), 52-week heatmap,
  day/hour histograms — one scan feeds all of them, data layer only for now
  (visual Stats-tab redesign stays in Sprint D's mockup-first cycle)
- src/stats/cadence.rs: release cadence (median tag gap, local tags, Tier A)
- src/stats/size.rs + cache.rs: git-object vs working-tree size, persisted
  in new ~/.vulp/stats_cache.toml (max 1 sample/day), recorded only during
  [R]/vulp refresh — confirms vulp's own filter-repo cleanup (539MB->834KB)
- src/digest.rs: WeeklyDigest struct + 3 renderers (Discord embed/markdown
  archive/plaintext); vulp digest [send discord|webhook <url>] CLI, webhook
  stored via existing auth token store (~/.vulp/.tokens, 0600), never logged;
  send respects --dry-run
- TUI: [d] on Stats previews the digest in a Modal::Info before anything is
  ever sent
- fix: list_contributors used --all (double-counts after fetching remotes),
  now --branches
- config: stats_author_emails field (TUI-editable)
- verified: streak/delta/top-repo/release math against a hand-built commit
  history, webhook validation+permissions, dry-run, real refresh (51 repos,
  vulp's own entry confirms the filter-repo win), TUI preview via pyte

src/config/mod.rs

@@ -48,6 +48,11 @@ pub struct AppConfig {
     /// CLI commands refuse to touch unless `--force` is passed.
     #[serde(default)]
     pub protected_repos:     Vec<String>,
+    /// Author emails counted as "you" for streak/heatmap/digest stats.
+    /// Empty = fall back to `git config user.email`, then `email`. GitHub's
+    /// no-reply address for each configured account is always matched too.
+    #[serde(default)]
+    pub stats_author_emails: Vec<String>,
 }
 
 fn default_license_kind() -> String { "mit".into() }
@@ -87,6 +92,7 @@ impl Default for AppConfig {
             license_holder:     String::new(),
             default_license:    default_license_kind(),
             protected_repos:    vec![],
+            stats_author_emails: vec![],
         }
     }
 }
@@ -113,6 +119,7 @@ impl AppConfig {
             ("license_holder",     self.license_holder.clone()),
             ("default_license",    self.default_license.clone()),
             ("protected_repos",    self.protected_repos.join(", ")),
+            ("stats_author_emails", self.stats_author_emails.join(", ")),
         ]
     }
 
@@ -143,6 +150,13 @@ impl AppConfig {
                     .filter(|s| !s.is_empty())
                     .collect();
             }
+            "stats_author_emails" => {
+                self.stats_author_emails = value
+                    .split(',')
+                    .map(|s| s.trim().to_string())
+                    .filter(|s| !s.is_empty())
+                    .collect();
+            }
             "workspace_groups"   => {
                 self.workspace_groups = value
                     .split(',')