feat: s3 storage client

feat: claude.md

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+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Project Overview
+
+Go service that extracts YAML frontmatter metadata from markdown files stored in cloud object storage (Backblaze B2 or AWS S3), then writes that metadata back as object attributes/metadata. Supports draft/prod workflow where draft files (`.draft.md`) are compared against prod (`.md`) and skipped if unchanged.
+
+## Commands
+
+```bash
+# Build
+go build -o metadata-extractor main.go
+
+# Run (requires B2_KEY_ID and B2_APPLICATION_KEY env vars, see .envrc)
+go run main.go -c config/config.json
+
+# No tests or linter configured yet
+```
+
+## Architecture
+
+Three-layer pipeline: **Config → Storage → Frontmatter parsing**
+
+- `main.go` — Entry point. Loads config, scans bucket, fans out goroutines (semaphore-bounded) to process each file: read → parse frontmatter → write metadata back as object attributes.
+- `config/config.go` — Loads JSON config with `os.ExpandEnv()` for credential injection. Validates via `validator/v10` struct tags. Storage type (`"b2"` or `"s3"`) selects which config struct and client to use.
+- `internal/storage/storage_interface.go` — `StorageClient` interface (`Scan`, `GetReader`, `WriteMetadata`, `CompareDraftAndProd`). Factory map registers all backends.
+- `internal/storage/b2.go` — Backblaze B2 implementation. Uses SHA1 for draft/prod change tracking. Writes metadata to B2 object `Info` map.
+- `internal/storage/s3.go` — AWS S3 implementation (AWS SDK v2). Uses ETag for draft/prod change tracking. Writes metadata as S3 user metadata. Supports custom `Endpoint` + `UsePathStyle` for S3-compatible stores (MinIO, etc).
+- `internal/frontmatter/parser.go` — Extracts content between `---` delimiters, unmarshals YAML into `Metadata` struct.
+
+## Key Conventions
+
+- Metadata keys use kebab-case (e.g., `short-description`, `action-date`) — stored in B2 `Info` map or S3 user metadata
+- Geolocation format: `"{x} {y}"` or `"{x} {y} {areaError}"` — space-separated floats, validated in both storage backends
+- Change tracking: B2 uses SHA1 (`metadata-last-update-sha1`), S3 uses ETag (`metadata-last-update-etag`)
+- Semantic commit messages: `feat:`, `fix:`, `refactor:`
+- Structured logging via `log/slog`