feat: reword phase to epic, update mkdocs

docs/content/stories/epic5/5.6-secret-store.md

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+# Story 5.6: Secret Store Integration
+
+## Metadata
+- **Story ID**: 5.6
+- **Title**: Secret Store Integration
+- **Epic**: 5 - Infrastructure Adapters
+- **Status**: Pending
+- **Priority**: Medium
+- **Estimated Time**: 4-5 hours
+- **Dependencies**: 0.2
+
+## Goal
+Implement secret store integration supporting HashiCorp Vault and AWS Secrets Manager for secure secret management.
+
+## Description
+This story implements secret store adapters that can retrieve secrets from external secret management systems, with integration into the configuration system.
+
+## Deliverables
+
+### 1. Secret Store Interface (`pkg/infra/secret/secret.go`)
+- `SecretStore` interface with:
+  - `GetSecret(ctx context.Context, key string) (string, error)`
+  - `GetSecrets(ctx context.Context, prefix string) (map[string]string, error)`
+
+### 2. Vault Implementation (`internal/infra/secret/vault_store.go`)
+- HashiCorp Vault client
+- Support KV v2 secrets
+- Authentication (token, app role)
+- Secret caching
+
+### 3. AWS Secrets Manager (`internal/infra/secret/aws_secrets.go`)
+- AWS Secrets Manager client
+- Secret retrieval
+- Secret caching
+
+### 4. Configuration Integration
+- Integrate with config loader
+- Overlay secrets on top of file/env config
+- Load secrets lazily (cache)
+- Secret key resolution
+
+### 5. Configuration
+- Secret store config in `config/default.yaml`:
+  - Provider (vault, aws, none)
+  - Connection settings
+  - Cache settings
+
+### 6. DI Integration
+- Provider function for SecretStore
+- Register in DI container (optional, via config)
+
+## Acceptance Criteria
+- [ ] Secret store interface is defined
+- [ ] Vault implementation works
+- [ ] AWS Secrets Manager implementation works
+- [ ] Secrets are loaded into config
+- [ ] Secret caching works
+- [ ] Configuration integration works
+- [ ] Secret store is optional (can be disabled)
+
+## Files to Create/Modify
+- `pkg/infra/secret/secret.go` - Secret store interface
+- `internal/infra/secret/vault_store.go` - Vault implementation
+- `internal/infra/secret/aws_secrets.go` - AWS implementation
+- `internal/config/loader.go` - Integrate secret loading
+- `internal/di/providers.go` - Add secret store provider
+- `config/default.yaml` - Add secret store config
+