feat(cli): [project] manifest block, --takeover, groups ls owner column

Makes §7 ownership usable end to end from the CLI.

- manifest: a `[project]` block (slug + optional name), independent of the
  [app]/[group] XOR; threaded onto every apply from the repo. --dir reads it
  from the tree ROOT's picloud.toml (a [project] elsewhere is ignored with a
  note).
- client: apply_node/apply_tree carry project + takeover; the 409 branch now
  surfaces the server message verbatim (covers StateMoved AND OwnershipConflict,
  both self-contained). New groups_list_with_owner captures the owner slug.
- pic apply --takeover flag; pic groups ls gains an `owner` column (the owning
  project's slug, or — when unclaimed).
- server: /api/v1/admin/groups now returns each group flattened with its owner
  slug (via list_with_owner) — the shared Group deserialize ignores the extra
  field, so pic groups tree / the dashboard are unaffected.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/picloud-cli/src/cmds/plan.rs

@@ -49,7 +49,7 @@ pub async fn run(manifest_path: &Path, env: Option<&str>, mode: OutputMode) -> R
 pub async fn run_tree(dir: &Path, env: Option<&str>, mode: OutputMode) -> Result<()> {
     let creds = config::resolve()?;
     let client = Client::from_creds(&creds)?;
-    let (bundle, _count) = crate::discover::build_tree(dir, env)?;
+    let (bundle, _count, _project) = crate::discover::build_tree(dir, env)?;
     let plan = client.plan_tree(&bundle).await?;
     if !plan.state_token.is_empty() {
         if let Err(e) = crate::linkstate::write_plan(dir, TREE_TOKEN_KEY, &plan.state_token) {