feat: add reusable egress policies for agents and sandboxes

[EItanya]

This PR adds reusable EgressPolicy resources so agents and standalone sandboxes can install dependencies or call APIs beyond their inferred model, MCP, and artifact access.

Agent and SandboxTemplate select policies through spec.egressPolicyRefs in the same namespace. Policies grant HTTP(S) origins, including wildcard reachability. Authentication uses one header, optional literal prefix, and Secret reference:

authentication:
  header: Authorization
  prefix: "Bearer "
  secretKeyRef:
    name: ticket-auth
    key: token

Credential injection requires an exact HTTPS origin. The gateway replaces a client-supplied placeholder header, and Secret values stay out of runtime revisions. Conflicting explicit and inferred credentials fail preparation.

Resolved grants are pinned to each revision and installed before the Actor resumes. Policy edits affect newly prepared revisions; they do not revoke existing sessions or sandboxes. The namespace is the sharing boundary for policies and credentials.

Includes CRDs, clients, Helm packaging, controller dependencies, UI preservation of policy refs, and usage/lifecycle documentation. Stored credential bindings now include scheme and port, so this assumes a clean install; old hostname-only bindings are rejected.

Validation: CRD generation and Kubernetes admission tests, translator/egress/Substrate and controller tests, PostgreSQL and lifecycle race tests, Go lint, UI typecheck, and changed-file ESLint passed. Live E2E coverage is added but has not run because KAGENT_E2E_API_URL is unset.

[blacksmith-sh]

Found 2 test failures on Blacksmith runners:

Failures

Test	View Logs
TestSessionEgressPolicyPinsAdditionalDestinations	View Logs
github.com/kagent-dev/kagent/go/core/test/e2e/ TestSessionEgressPolicyPinsAdditionalDestinations	View Logs

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