Phase 5 C2. A `picloud.toml` can now declare a `[group]` node (instead of
`[app]`): the group's own scripts + `[vars]` (+ secret names). `pic plan` /
`pic apply` reconcile it via the C1 group endpoints — the same plan/apply/
bound-token flow as an app, routed by node kind.
* `Manifest` is now app-XOR-group: `app`/`group` are both optional with an
exactly-one check in `parse`, plus a group-node guard that rejects
`[[routes]]`/`[[triggers]]` (those bind to an app). Accessors `slug()` /
`is_group()` replace the hardcoded `manifest.app.slug`.
* `client`: `plan_node`/`apply_node` take a `NodeKind { App | Group }` that
selects the `apps` vs `groups` API path; the old `plan`/`apply` wrappers
are gone (callers pass the kind).
* `pic plan`/`apply` detect the node kind from the manifest and label output
`group`/`app`; `pic config --effective` cleanly rejects a group manifest
(effective config is an app's inherited view).
Live-validated: a `[group]` manifest with a script + var → `pic plan` (script
+ var creates) → `pic apply` (group-owned) → idempotent re-plan noop →
`pic scripts ls --group` shows it. Manifest unit test for group parse +
app-only-block rejection; init/pull/plan/apply/overlay journeys all green.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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