feat(apply): declarative extension_points reconcile (§5.5 C3)

Thread extension-point markers through the manifest and the apply engine —
a name-only resource modeled on `secrets` (shape) with `vars`-style
create/prune write behavior.

- manifest: top-level `extension_points = ["theme", …]` (allowed on app and
  group; overlay stays base-only via deny_unknown_fields); `build_bundle`
  emits the names.
- apply_service: `Bundle.extension_points`, `CurrentState.extension_point_names`,
  `Plan.extension_points` (+ is_noop), `diff_extension_points`
  (declared→Create / live-undeclared→Delete / else NoOp), `load_current`
  loads them, `reconcile_node_tx` inserts on Create + deletes on prune,
  `state_token_with_names` folds in `ep|<name>`, `validate_bundle` rejects
  duplicates + reserved names, `ApplyReport` gains created/deleted counts.
- CLI client + plan/apply rendering: `extension_points` in PlanDto/NodePlanDto/
  ApplyReportDto, an `extension_point` row group in `pic plan`, a count in the
  apply summary.

pull sets it empty for now (wired to the read endpoint in C4).

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

crates/picloud-cli/src/manifest.rs

@@ -49,6 +49,12 @@ pub struct Manifest {
     /// The overlay merges per-env, last-write-wins by key.
     #[serde(default, skip_serializing_if = "BTreeMap::is_empty")]
     pub vars: BTreeMap<String, toml::Value>,
+    /// `extension_points = ["theme", …]` (§5.5) — module names this node marks
+    /// as provided/overridable by descendants. Name-only, like `[secrets]`; the
+    /// optional default body is a co-located `[[scripts]]` module of the same
+    /// name. Allowed on app and group nodes.
+    #[serde(default, skip_serializing_if = "Vec::is_empty")]
+    pub extension_points: Vec<String>,
 }
 
 impl Manifest {
@@ -488,6 +494,7 @@ mod tests {
                 ("region".to_string(), toml::Value::String("eu".into())),
                 ("max-retries".to_string(), toml::Value::Integer(3)),
             ]),
+            extension_points: vec!["theme".into()],
         }
     }
 
@@ -591,12 +598,14 @@ mod tests {
             triggers: ManifestTriggers::default(),
             secrets: ManifestSecrets::default(),
             vars: BTreeMap::new(),
+            extension_points: vec![],
         };
         let text = m.to_toml().unwrap();
         assert!(!text.contains("[[scripts]]"), "got:\n{text}");
         assert!(!text.contains("triggers"), "got:\n{text}");
         assert!(!text.contains("secrets"), "got:\n{text}");
         assert!(!text.contains("vars"), "got:\n{text}");
+        assert!(!text.contains("extension_points"), "got:\n{text}");
         // Still round-trips.
         assert_eq!(m, Manifest::parse(&text).unwrap());
     }