[Breaking] Rename tmp config section to storage

[michaelmcnees]

Problem

The tmp config section (tmp.type, tmp.bucket, tmp.path, tmp.retention) stores execution artifacts — screenshots, PDFs, reports — with configurable retention. These are not temporary files. The naming is misleading and will confuse users as the artifact system matures.

Proposed Solution

Rename the config section from tmp to storage:

# Before
tmp:
  type: s3
  bucket: mantle-artifacts
  prefix: artifacts/
  retention: 24h

# After
storage:
  type: s3
  bucket: mantle-artifacts
  prefix: artifacts/
  retention: 24h

Environment variables change accordingly:

• MANTLE_TMP_TYPE → MANTLE_STORAGE_TYPE
• MANTLE_TMP_BUCKET → MANTLE_STORAGE_BUCKET
• MANTLE_TMP_PREFIX → MANTLE_STORAGE_PREFIX
• MANTLE_TMP_PATH → MANTLE_STORAGE_PATH
• MANTLE_TMP_RETENTION → MANTLE_STORAGE_RETENTION

Scope

• Rename TmpConfig struct to StorageConfig in internal/config/config.go
• Update Viper bindings and env var prefixes
• Rename TmpStorage interface/field references in internal/artifact/
• Update docker-compose.yml environment variables
• Update documentation (configuration.md, deployment-guide.md, artifact docs)
• Document the breaking change in the changelog

Context

This is a breaking config change — better to do it now while the user base is small than after v1.0.0. Related to #52 (config file versioning) which could ease future migrations.