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>
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