docs(dev): add development model, delivery strategy, and reviewer workflow

Context

This MR formalizes the delivery approach for the Artifact Registry MVP. It documents the development model (how we organize work), the delivery strategy (what ships when and why), and the reviewer workflow (which was missing from the agentic development guide).

Timeline estimates and engineer pairing assignments are intentionally out of scope. The delivery model outlined here is our best approach at maximizing throughput while maintaining tight quality requirements. Operational decisions like scheduling and pairing belong with engineering management and will be informed by real cycle-time data as the process progresses.

Summary

• docs/dev/development-model.md: principles, review model, MR sizing, platform foundations vs. vertical slices, cross-spec dependency protocol, minimum foundations for format work, delivery sequencing, and team allocation
• docs/dev/agentic-development.md: add reviewer workflow (AI pre-review, human L3 review, follow-up), update operator Phase 3 to reference /review-branch
• docs/specs/README.md: add Type column, reclassify S19-S24 as Platform
• .claude/skills/spec-author/SKILL.md: align cross-spec dependency guidance with development model
• .claude/skills/planner/SKILL.md: add 200-400 LOC target to MR breakdown