Release v0.1.2: namespace-aware plugin deployment

[guimou]

Summary

• Namespace-aware installs: plugins can now declare a target namespace in their plugin.yaml; a confirmation modal is shown before installing to a custom namespace
• Helm chart namespace management: the chart conditionally creates its own Namespace resource when it differs from the release namespace; all defaults now use cp-plugins-admin
• BFF runtime namespace: the BFF pod namespace is injected via Kubernetes Downward API and exposed through GET /api/config as bffNamespace, enabling future cross-namespace resource creation without hardcoded values
• SSE heartbeat: lifecycle SSE endpoints now send keepalive heartbeats, and the Helm timeout is aligned to prevent premature connection drops
• Migration guide: documentation added for users upgrading from pre-0.1.2 deployments that used the old namespace

Test plan

• Verify helm template renders POD_NAMESPACE env var with Downward API fieldRef
• Verify GET /api/config returns bffNamespace alongside existing fields
• Install a plugin with a custom namespace declared in its plugin.yaml and confirm the install modal appears
• Install a plugin without a custom namespace and confirm no modal is shown
• Run npm test (frontend) and cd bff && npm test (BFF) — all tests pass
• Run helm lint chart/ and helm template test chart/ — no errors