feat(codeaf): add CodeAF coding agent package

CodeAF (Agent-Field/CodeAF) is a Go coding harness that ships one binary.
Pinned to v0.7.1; the flake's nixpkgs already carries go 1.26.7 and go.mod
asks for 1.26.5, so no extra nixpkgs input is needed.

Two things about this build are not obvious from the derivation:

- `make build` runs two host-side steps before compiling: `go generate
  ./internal/manual` packs the built-in manual into the pages.pack.gz and
  chat.pack.gz that the codeaf_packed_manual tag embeds, and
  internal/furrowbin/cmd/fetch stages the pinned Agent-Field/furrow release
  into internal/furrowbin/cache for go:embed. Both are run in preBuild with
  GOOS/GOARCH unset, the way the Makefile does it, so a cross build does not
  try to run a target binary on the build machine. buildGoModule runs preBuild
  in the vendoring derivation as well, before vendor/ exists and before the
  dependencies are in reach, so the two steps are gated on vendor/.
- The furrow step gets its bytes from fetchurl and `-from`, the offline road
  upstream documents. The fetcher still checks the sha256 in
  internal/furrowbin/pin.json, so the hashes here duplicate a pin that is
  already in the source; drop the furrow entirely and codeaf still builds and
  falls back to looking for the binary on PATH.

Tests are off: the upstream suite is a make/CI matrix with network and timing
assumptions, not a `go test ./...` that fits buildGoModule.

Verified with nix build .#codeaf, `codeaf --version` reporting 0.7.1 and
`codeaf manual` listing the packed manual pages.

Refs nix-overlay-m5y

README.md

@@ -14,6 +14,7 @@ A custom Nix overlay and flake providing additional packages not found in upstre
 |---------|-------------|----------|
 | `awg-tool` | CLI for AmneziaWG self-hosting — generates 1.0/1.5/2.0/3.0 obfuscation parameters, exports .conf and vpn:// configs, installs servers over SSH | Networking |
 | `aionui` | Free, open-source, Cowork app with AI Agents | AI Coding Agents |
+| `codeaf` | Coding harness and software factory for open models: hand work off, then watch every project from one terminal | AI Coding Agents |
 | `container-use` | Containerized environments for coding agents | AI Coding Agents |
 | `desloppify` | Multi-language codebase health scanner and technical debt tracker for AI agents | AI Coding Agents |
 | `ds4` | DeepSeek 4 Flash and PRO local inference engine for ROCm (Strix Halo) | AI Inference |
@@ -185,6 +186,7 @@ nix-overlay/
 ├── packages/                    # Package definitions
 │   ├── awg-tool/               # AmneziaWG parameter generation and SSH deployment CLI
 │   ├── aionui/                 # AionUi - AI Cowork desktop app
+│   ├── codeaf/                 # Coding harness and software factory for open models
 │   ├── container-use/          # Containerized environments for coding agents
 │   ├── default/                # Meta-package listing all packages
 │   ├── desloppify/             # Codebase health scanner for AI agents