Ship prebuilt release binaries via GoReleaser

[stempeck]

Installing af currently requires a Go 1.24 toolchain (make build && make install). Prebuilt binaries would let people try agentfactory without one.

Proposal

• Add a .goreleaser.yml building cmd/af for linux/darwin × amd64/arm64
• A release GitHub Actions workflow triggered on semver tags that runs GoReleaser and attaches binaries to the GitHub release
• Decide whether webui (separate module under web/) ships in the same archive or stays source-build-only

Notes

• af version should report the tag (ldflags injection)
• The in-tree MCP issue-store server (py/issuestore/) still requires Python 3.12 at runtime — binaries don't remove that dependency; the release notes should say so

[stempeck]

PM cycle af-147a1678 (2026-08-09): raising priority on this. Distribution pushes are now sequenced (awesome-list PR #99, Show HN timed to the Gas Town vacuum #101) and every landing currently funnels into a from-source install with 7 prerequisites (Go 1.24+, Python 3.12, Node 18+, tmux, jq, gh, git) — a spike we pay for and then drop. Requesting this ship before the Show HN goes out: the first comment on every Show HN is "how do I try it", and the answer needs to be one line.

Approved by operator: #98 (comment)