docs: add CLAUDE.md project instructions

Co-Authored-By: Virgil <virgil@lethean.io>

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.
+
+## Build & Test Commands
+
+This project uses the Core CLI (`core` binary), not `go` directly.
+
+```bash
+core go test                          # run all tests
+core go test --run TestConfig_Get_Good  # run a single test
+core go cov                           # test with coverage
+core go cov --open                    # coverage + open HTML report
+
+core go qa                            # format, vet, lint, test
+core go qa full                       # adds race detector, vuln scan, security audit
+
+core go fmt                           # format
+core go vet                           # vet
+core go lint                          # lint
+```
+
+This is a library package — there is no binary to build or run.
+
+## Architecture
+
+**Dual-Viper pattern**: `Config` holds two `*viper.Viper` instances:
+- `v` (full) — file + env + defaults; used for all reads (`Get`, `All`)
+- `f` (file-only) — file + explicit `Set()` calls; used for persistence (`Commit`)
+
+This prevents environment variables from leaking into saved config files. When implementing new features, maintain this invariant: writes go to both `v` and `f`; reads come from `v`; persistence comes from `f`.
+
+**Resolution priority** (ascending): defaults → file → env vars (`CORE_CONFIG_*`) → `Set()`
+
+**Service wrapper**: `Service` in `service.go` wraps `Config` with framework lifecycle (`core.Startable`). Both `Config` and `Service` satisfy `core.Config`, enforced by compile-time assertions.
+
+**Storage abstraction**: All file I/O goes through `io.Medium` (from `go-io`). Tests use `io.NewMockMedium()` with an in-memory `Files` map — never touch the real filesystem.
+
+## Conventions
+
+- **UK English** in comments and documentation (colour, organisation, centre)
+- **Error wrapping**: `coreerr.E(caller, message, underlying)` from `go-log`
+- **Test naming**: `_Good` (happy path), `_Bad` (expected errors), `_Ugly` (panics/edge cases)
+- **Functional options**: `New()` takes `...Option` (e.g. `WithMedium`, `WithPath`, `WithEnvPrefix`)
+- **Conventional commits**: `type(scope): description`
+- **Go workspace**: module is part of `~/Code/go.work`
+
+## Dependencies
+
+- `forge.lthn.ai/core/go-io` — `Medium` interface for storage
+- `forge.lthn.ai/core/go-log` — `coreerr.E()` error helper
+- `forge.lthn.ai/core/go/pkg/core` — `core.Config`, `core.Startable`, `core.ServiceRuntime` interfaces
+- `github.com/spf13/viper` — configuration engine
+- `github.com/stretchr/testify` — test assertions