Sprint B.6: CLI guardrails — --dry-run, protected_repos, VULP_AGENT marker

- global --dry-run: sync/release/init/collab/license print 'DRY RUN: would...'
  and exit 0 (no auth needed for previews where possible)
- config protected_repos (name/basename, case-insensitive): mutating commands
  exit 1 on listed repos unless --force; checked before dry-run
- VULP_AGENT env: activity::log centrally appends '· via agent' to detail,
  covering both CLI and TUI writers
- sync: unknown --action now rejected before touching any repo

src/cli.rs

@@ -19,6 +19,13 @@ pub struct Cli {
     /// Machine-readable JSON output on stdout (list/status/stats/... commands)
     #[arg(long, global = true)]
     pub json: bool,
+    /// Print what a mutating command (sync/release/init/collab/license)
+    /// would do without doing it
+    #[arg(long, global = true)]
+    pub dry_run: bool,
+    /// Proceed on repos listed in config `protected_repos`
+    #[arg(long, global = true)]
+    pub force: bool,
 }
 
 #[derive(Subcommand)]