Add gen/<generator>/ directory convention to policy

Summary

Adds a new "Generated Files" section to the QC policy document (Section 2.5) establishing the gen/<generator>/ directory convention.

Changes

• Directory structure: all generated files MUST go under gen/<generator-name>/ (e.g. gen/nix/, gen/haskell/)
• Formatting exclusion: single gen/** pattern replaces per-file exclusion lists
• Generator contract: generators must write to gen/, produce treefmt-conformant output, and not write outside gen/ without justification
• Version control: generated files are committed, not gitignored

Context

Requested by @bossypants in nix/horizon-flake-parts#16 as QC requirement R1 — document the convention before implementation begins.

Satisfies action item #2 (closed) from the synthesised plan.

cc @locallycompact for approval