Add cue to prerequisites and consolidate validation calls

[chicks-net]

Minor improvements identified across reviews:

• `cue` missing from prerequisites (5/15 PRs) - Add to `.just/lib/install-prerequisites.sh`
• `cue-verify` exit code inconsistency (6/15 PRs) - Returns 22 (curl) vs 1 (`gh api`). Normalize to 1
• `web_url` regex too permissive (7/15 PRs) - `docs/repo-toml.cue:24` accepts invalid URLs
• Redundant `cue export` calls - Consolidate to run once and cache result

Identified in code reviews across 15 PRs using gh-process v5.8.