docs: update gateway docs for the split-gateway architecture

[atemate]

What

Reconciles the broader gateway-architecture documentation with the current
split-gateway reality. The gateway was rearchitected from a single binary
with api/mesh modes and mandatory PostgreSQL into split containers — all from
the one asya-gateway image, selected via command::

• mesh-api (default entrypoint, ports 8080 external / 8081 internal)
• mcp-adapter (./mcp-adapter, :8082)
• a2a-adapter (./a2a-adapter, :8083)
• state-proxy-mesh sidecar for task/mesh state

State is pluggable via stateProxy.mesh.backend: pg-kv (PostgreSQL, default) or
pvc-kv (DuckDB-over-JSON on a PVC / in-memory) — PostgreSQL is no longer
mandatory. ASYA_GATEWAY_MODE and the old asya-gateway-api / asya-gateway-mesh
mode-based deployments/services are gone; services are now asya-gateway-mesh-api
(8080), -mesh-api-int (8081), -mcp (8082), -a2a (8083).

Independent of #507

This PR is independent of #507 (flow-registration mechanism). It does not
touch docs/setup/guide-gateway.md or docs/usage/guide-agentic-patterns.md —
those are owned by #507. Where these docs reference flow registration, they
cross-link to guide-gateway.md rather than re-documenting it.

Files updated

• docs/reference/components/core-gateway.md — biggest edit: api/mesh modes →
  split containers; PostgreSQL → pg-kv OR pvc-kv; corrected service
  names/ports, endpoints (/api/v1/mesh/...), state-ownership diagram, and
  auth (A2A only; MCP auth / OAuth 2.1 / /tools/call removed in code).
• docs/reference/env-vars.md — replaced the stale single-binary asya-gateway
  section (removed ASYA_GATEWAY_MODE, ASYA_DATABASE_URL, ASYA_GATEWAY_PORT,
  ASYA_CONFIG_*, DB-pool vars, MCP OAuth vars) with a pointer to the per-binary
  sections; added ASYA_BACKSTOP_INTERVAL and a pvc-kv state-proxy section.
• docs/concepts/http-gateway.md — two deployment modes → split containers;
  pluggable state backend.
• docs/reference/credentials.md — gateway DB secret model corrected (pg-kv reads
  DB_PASSWORD via database.existingSecret; no DSN secret / init container;
  pvc-kv needs none); a2a.auth.apiKey.
• docs/setup/guide-pause-resume.md — task-status URL → asya-gateway-mesh-api
  service + /api/v1/mesh/<id> path.
• docs/setup/start-gcp-gke.md — split-gateway install with pvc-kv (no-Postgres)
  and pg-kv options, database.* values, Ingress instead of LoadBalancer,
  asya-gateway-mcp / -a2a services, dropped OAuth claim, /a2a/hello → /a2a/,
  removed stale sidecar.gatewayURL flag.

Key corrections

• api/mesh modes → split containers (one image, command:-selected)
• PostgreSQL now optional via pvc-kv
• service-name / port updates (asya-gateway-mesh-api[-int], -mcp, -a2a)
• removed-in-code endpoints/auth dropped: OAuth 2.1, /tools/call,
  /mesh/config-reload, MCP auth wiring

Verification

• All env vars, endpoints, service names, and Helm values verified against the
  current chart (deploy/helm-charts/asya-gateway/), the e2e profiles
  (sqs-s3-pvc.yaml, pubsub-gcs-pg.yaml), and the gateway source
  (src/asya-gateway/cmd/{mesh-api,mcp-adapter,a2a-adapter}).
• All internal doc links resolve; pre-commit hooks pass.

Intentionally left stale (out of scope)

• docs/reference/specs/gateway-api.md — still lists /tools/call, /oauth/*,
  /mesh/config-reload, /stream/{id}, "two deployments". It did not match
  the staleness grep and is a large spec deserving its own follow-up.
• src/asya-gateway/README.md — source README (not under docs/), still
  describes the single binary.

[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

Results:
0 new issues

View in Codacy

_{NEW Get contextual insights on your PRs based on Codacy's metrics, along with PR and Jira context, without leaving GitHub. Enable AI reviewer
TIP This summary will be updated as you push new changes.}

[gemini-code-assist]

Code Review

This pull request refactors the asya-gateway architecture, splitting it from a single binary into separate containers (mesh-api, mcp-adapter, a2a-adapter, and state-proxy-mesh sidecar) within a single pod. It also introduces pluggable state backends, adding a lightweight pvc-kv option that eliminates the mandatory PostgreSQL requirement. The documentation, environment variables, and setup guides have been updated to reflect this split. Feedback on the changes suggests correcting outdated references to asya-gateway in the transport environment variables documentation, which should instead point to asya-sidecar.

_{Note: Security Review has been skipped due to the limited scope of the PR.}

[gemini-code-assist]

The heading ### Transport (same as gateway) and the description on lines 158-161 still reference asya-gateway. However, the asya-gateway transport environment variables section has been removed in this PR. Please update this section to reference asya-sidecar instead (e.g., ### Transport (same as sidecar) and Uses the same queue transport env vars as asya-sidecar...).