docs: add project specs and documentation for Modbus relay control

Initialize project documentation structure:
- Add CLAUDE.md with development guidelines and architecture principles
- Add project constitution (v1.1.0) with hexagonal architecture and SOLID principles
- Add MCP server configuration for Context7 integration

Feature specification (001-modbus-relay-control):
- Complete feature spec for web-based Modbus relay control system
- Implementation plan with TDD approach using SQLx for persistence
- Type-driven development design for domain types
- Technical decisions document (SQLx over rusqlite, SQLite persistence)
- Detailed task breakdown (94 tasks across 8 phases)
- Specification templates for future features

Documentation:
- Modbus POE ETH Relay hardware documentation
- Modbus Application Protocol specification (PDF)

Project uses SQLx for compile-time verified SQL queries, aligned with
type-driven development principles.

specs/templates/plan-template.md

@@ -0,0 +1,104 @@
+# Implementation Plan: [FEATURE]
+
+**Branch**: `[###-feature-name]` | **Date**: [DATE] | **Spec**: [link]
+**Input**: Feature specification from `/specs/[###-feature-name]/spec.md`
+
+**Note**: This template is filled in by the `/speckit.plan` command. See `.specify/templates/commands/plan.md` for the execution workflow.
+
+## Summary
+
+[Extract from feature spec: primary requirement + technical approach from research]
+
+## Technical Context
+
+<!--
+  ACTION REQUIRED: Replace the content in this section with the technical details
+  for the project. The structure here is presented in advisory capacity to guide
+  the iteration process.
+-->
+
+**Language/Version**: [e.g., Python 3.11, Swift 5.9, Rust 1.75 or NEEDS CLARIFICATION]  
+**Primary Dependencies**: [e.g., FastAPI, UIKit, LLVM or NEEDS CLARIFICATION]  
+**Storage**: [if applicable, e.g., PostgreSQL, CoreData, files or N/A]  
+**Testing**: [e.g., pytest, XCTest, cargo test or NEEDS CLARIFICATION]  
+**Target Platform**: [e.g., Linux server, iOS 15+, WASM or NEEDS CLARIFICATION]
+**Project Type**: [single/web/mobile - determines source structure]  
+**Performance Goals**: [domain-specific, e.g., 1000 req/s, 10k lines/sec, 60 fps or NEEDS CLARIFICATION]  
+**Constraints**: [domain-specific, e.g., <200ms p95, <100MB memory, offline-capable or NEEDS CLARIFICATION]  
+**Scale/Scope**: [domain-specific, e.g., 10k users, 1M LOC, 50 screens or NEEDS CLARIFICATION]
+
+## Constitution Check
+
+*GATE: Must pass before Phase 0 research. Re-check after Phase 1 design.*
+
+[Gates determined based on constitution file]
+
+## Project Structure
+
+### Documentation (this feature)
+
+```text
+specs/[###-feature]/
+├── plan.md              # This file (/speckit.plan command output)
+├── research.md          # Phase 0 output (/speckit.plan command)
+├── data-model.md        # Phase 1 output (/speckit.plan command)
+├── quickstart.md        # Phase 1 output (/speckit.plan command)
+├── contracts/           # Phase 1 output (/speckit.plan command)
+└── tasks.md             # Phase 2 output (/speckit.tasks command - NOT created by /speckit.plan)
+```
+
+### Source Code (repository root)
+<!--
+  ACTION REQUIRED: Replace the placeholder tree below with the concrete layout
+  for this feature. Delete unused options and expand the chosen structure with
+  real paths (e.g., apps/admin, packages/something). The delivered plan must
+  not include Option labels.
+-->
+
+```text
+# [REMOVE IF UNUSED] Option 1: Single project (DEFAULT)
+src/
+├── models/
+├── services/
+├── cli/
+└── lib/
+
+tests/
+├── contract/
+├── integration/
+└── unit/
+
+# [REMOVE IF UNUSED] Option 2: Web application (when "frontend" + "backend" detected)
+backend/
+├── src/
+│   ├── models/
+│   ├── services/
+│   └── api/
+└── tests/
+
+frontend/
+├── src/
+│   ├── components/
+│   ├── pages/
+│   └── services/
+└── tests/
+
+# [REMOVE IF UNUSED] Option 3: Mobile + API (when "iOS/Android" detected)
+api/
+└── [same as backend above]
+
+ios/ or android/
+└── [platform-specific structure: feature modules, UI flows, platform tests]
+```
+
+**Structure Decision**: [Document the selected structure and reference the real
+directories captured above]
+
+## Complexity Tracking
+
+> **Fill ONLY if Constitution Check has violations that must be justified**
+
+| Violation | Why Needed | Simpler Alternative Rejected Because |
+|-----------|------------|-------------------------------------|
+| [e.g., 4th project] | [current need] | [why 3 projects insufficient] |
+| [e.g., Repository pattern] | [specific problem] | [why direct DB access insufficient] |