[P0] Add stack E2E proving contract-shaped council execution

[tgarciai]

Part of #31.

Goal:
Add a reproducible stack E2E proving that Choreographer can execute the bounded, validated flow PIR will later depend on.

Target proof:

bounded external trigger or RPC
  -> structured external context bundle
    -> real council
      -> validated structured result
        -> runtime execution or bounded outbound result

Scope:

• use real provider-backed councils, not noop-only paths
• use the structured context / output surfaces introduced by readiness work
• exercise metadata propagation and validation in the same run
• keep the E2E reproducible locally and in cluster-oriented manual execution flows

Acceptance criteria:

• one green E2E run proves context -> council -> validated result -> execution / bounded output
• the E2E fails deterministically when validation or execution fails
• the scenario is documented as a readiness gate for PIR integration

Tests:

• compose or local stack E2E
• kubernetes / cluster-oriented manual E2E variant where applicable