feat: annotation categories and severity tags

[viv]

Summary

Add optional annotation categories/severity tags so reviewers can signal priority and agents can triage intelligently.

Problem

All annotations look identical regardless of importance. A "typo in footer" and a "broken auth flow" get equal visual weight. Reviewers can't signal priority, and agents can't triage — they process annotations in arbitrary order.

Proposal

• Add an optional category field to annotations with a short, fixed list (max 5): Bug, Content, Style, A11y, Question
• Display as coloured pills/tags in the panel and on page highlights
• Selectable via a dropdown or pill selector in the annotation creation popup
• Default is no category — preserving zero-friction creation for reviewers who don't want to categorise
• Expose the category field via MCP tools (list_annotations, start_work) for agent filtering and triage
• Support optional category filter parameter on list_annotations

Key Constraints

• Must stay strictly optional — no category is a valid default; the quick-annotation workflow must not be slowed down
• Minimal taxonomy — max 5 categories, not configurable per-project. Resist scope creep into a full issue tracker
• Zero-config principle — no configuration needed; the categories are built-in and universal
• Aligns with the existing data model pattern (optional fields with sensible defaults)

Complexity

Low-Medium

Consensus

3/3 STRONG YES — all three perspectives independently proposed this feature during brainstorming.

Related

• Part of feature roadmap — brainstormed and debated by agent panel #62 (feature roadmap — Tier 1)