docs: design the portable solution architect agent

Record the direction and build plan for replacing the upstream
google-cloud-solution-architecture skill with a portable agent.

- ADR-0002: Agent Skills are the portable unit of behaviour, loaded by
  the framework's native skill runtime; discovery and grounding use
  each Cloud provider's hosted remote MCP servers.
- ADR-0003: LangGraph holds Execution state, checkpoints and
  interrupts; a ReAct Orchestrator and Specialists run as ADK
  LlmAgents with SkillToolset. Supersedes ADR-0001.
- CONTEXT.md: domain glossary (Execution, Phase, Approval, Revision,
  Orchestrator, Specialist, Deliverable, Dependency, Cloud provider).
- .scratch/solution-architect-agent/spec.md: build spec with the
  dependency graph, Revision rules, tool tiers, MCP allowlists, A2A
  interaction, tests and nine build increments.
- .scratch/phase-pipeline/spec.md: superseded; kept as decision log.
- CLAUDE.md and docs/agents/: agent skill configuration (local
  markdown issue tracker, triage labels, domain docs).
- README.md: direction note; flags current ADK classes as stubs.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

README.md

@@ -2,6 +2,12 @@
 
 A production-ready Google Cloud Solution Architecture Agent built using **Google ADK (Agent Development Kit)**, **LangChain**, and **LangGraph**, featuring a multi-agent orchestration graph with iterative review loops, dynamic local skill loading, PostgreSQL database persistence, a Starlette REST API server, and an offline evaluation harness.
 
+> **Direction and current state (2026-09-15).** This agent is being built to replace the [`google-cloud-solution-architecture`](https://github.com/google/skills/blob/main/skills/cloud/google-cloud-solution-architecture/SKILL.md) skill with something that runs on any agent framework and, later, targets any Cloud provider. Google Cloud is the first provider.
+> - **Target:** behaviour lives in [Agent Skills](https://agentskills.io/specification), loaded by a framework's own skill runtime (Google ADK `SkillToolset`, or LangChain Deep Agents skills). This repo supplies the tooling those Skills call. See [ADR-0002](docs/adr/0002-skills-are-the-portable-unit-of-behaviour.md).
+> - **Today:** the `google.adk` classes below are local compatibility stubs. The real `google-adk` package isn't used, and Phase outputs are fixed templates, not model-generated.
+>
+> Vocabulary is defined in [CONTEXT.md](CONTEXT.md); decisions are recorded in [docs/adr/](docs/adr/).
+
 ---
 
 ## 🌟 Architecture & Features