feat(sidecar): add signed Go 1.27 policy decision service

[usernamenenad]

Summary

Introduce the first Go 1.27 decision-sidecar vertical slice for the Aegisora v1alpha1 governance contract, rebased onto the latest upstream main after Aegisora 2.0.0 and its delegation/collaboration hardening.

This PR intentionally establishes a decision service before the controlled-proxy boundary. It validates execution intents, verifies and atomically activates signed policy snapshots, evaluates deterministic ALLOW / BLOCK / ESCALATE outcomes, records correlated audit events, and exposes policy-backed readiness.

The upstream v1alpha1 OpenAPI contract and semantic fixture corpus remain unchanged, so the sidecar is contract-compatible. The documentation now also accounts for the canonical ToolRegistry, scoped single-use authorization receipts, correlation integrity, delegated-agent capabilities, scope containment, and collaboration ownership boundaries in the current runtime.

What changed

• add the Go 1.27 sidecar service, Docker build, Makefile, local signing tools, examples, and documentation;
• implement strict v1alpha1 domain validation and load the repository's shared semantic fixtures in Go tests;
• implement Ed25519 policy signing and verification, canonical SHA-256 digests, expiry checks, scope checks, immutable snapshots, and atomic activation;
• implement a deliberately small deny-overrides policy dialect (BLOCK > ESCALATE > ALLOW);
• add bounded request idempotency with conflicting-payload rejection and concurrency-safe same-ID serialization;
• bind cached decisions to the active policy version and digest so policy replacement, removal, or expiry forces reevaluation;
• deep-copy cached risk signals and in-memory audit payloads so callers cannot mutate stored decisions or evidence;
• add strict JSON, body-size, content-type, header-timeout, and total request-read-timeout handling;
• add health, readiness, and active-policy metadata endpoints;
• add Go 1.27 CI and release verification;
• make the security-readiness workflow release-independent by deriving the expected public-package version from the canonical package manifest and scanning only Git-tracked repository content;
• update the repository investigation and architecture proposal for the current 2.0 runtime authority, delegation, and collaboration model.

Security behavior

• no valid unexpired policy means not-ready and fail-closed decisions;
• unsigned, modified, expired, or incorrectly scoped bundles cannot activate;
• allowed is mechanically derived from decision == ALLOW;
• duplicate request IDs replay only for identical intent payloads under the same active policy version and digest;
• policy changes invalidate compatible cache entries, preventing a stale cached ALLOW from surviving loss of authorization;
• a request-ID payload conflict returns BLOCK;
• broad allow rules cannot override a matching block rule;
• audit append failure prevents a successful decision response;
• bounded reads protect the local HTTP surface from oversized and indefinitely slow request bodies.

Verification

• make fmt-check vet test-race build tools
• npx --yes pnpm@11.20.0 verify
• all 85 TypeScript runtime tests pass, including delegation, expiry, revocation, scope-containment, sibling-isolation, collaboration-ownership, governance-boundary, and correlation-integrity traces
• git diff --check

Deliberate follow-up PRs

This PR does not claim a complete production enforcement boundary. The documented next slices are:

1. controlled upstream proxying with strict signed route selection and zero-side-effect adversarial tests;
2. authenticated workload identity over a Unix-domain-socket/local transport;
3. FastAPI policy polling, rollback protection, key rotation, and last-known-good persistence;
4. durable audit WAL/export and escalation approval flow;
5. TypeScript distributed mode with no ungoverned fallback.