Document our software factory approach #2497

Since v2.0.0 the repo moved to a maintainer + AI-agent production model — external contributors file issues, maintainers approve and prioritize on the board, and a coding agent takes each approved issue to a merged PR. That process is fully covered by AGENTS.md and .claude/skills/, but there's no single doc that explains the shape of the whole thing end to end for someone encountering it for the first time: the rules/skills split, the issue lifecycle, the PR flow, the automated dependency/security sweeps that replaced Dependabot, the quality gate that makes unsupervised merges safe, and where it's headed.

Add a doc (docs/ai-software-factory.md) that gives an outside reader that whole picture, backed by verifiable numbers (PRs merged, releases shipped, share of PRs that are agent-authored) rather than just describing the intent.