docs: add mkdocs, update links, add architecture documentation

docs/content/adr/0004-configuration-management.md

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+# ADR-0004: Configuration Management Library
+
+## Status
+Accepted
+
+## Context
+The platform needs a configuration system that:
+- Supports hierarchical configuration (defaults → files → env → secrets)
+- Handles multiple formats (YAML, JSON, env vars)
+- Provides type-safe access to configuration values
+- Supports environment-specific overrides
+- Can integrate with secret managers (future)
+
+Options considered:
+1. **spf13/viper** - Comprehensive configuration management
+2. **envconfig** - Environment variable only
+3. **koanf** - Lightweight configuration library
+4. **Standard library + manual parsing** - No external dependency
+
+## Decision
+Use **spf13/viper** (v1.18.0+) with **spf13/cobra** (v1.8.0+) for configuration management.
+
+**Rationale:**
+- Industry standard for Go configuration management
+- Supports multiple sources (files, env vars, flags)
+- Hierarchical configuration with precedence rules
+- Easy integration with Cobra for CLI commands
+- Well-documented and widely used
+- Supports future secret manager integration
+
+## Consequences
+
+### Positive
+- Flexible configuration loading from multiple sources
+- Easy to add new configuration sources
+- Type-safe access methods
+- Environment variable support via automatic env binding
+
+### Negative
+- Additional dependency
+- Viper can be verbose for simple use cases
+- Some learning curve for advanced features
+
+### Implementation Notes
+- Install: `go get github.com/spf13/viper@v1.18.0` and `github.com/spf13/cobra@v1.8.0`
+- Create `pkg/config/config.go` interface to abstract Viper
+- Implement `internal/config/viper_config.go` as concrete implementation
+- Load order: `default.yaml` → `development.yaml`/`production.yaml` → env vars → secrets (future)
+- Use typed getters (GetString, GetInt, GetBool) for type safety
+