Move testing detail into a verify-changes skill and tip rubric into TipScore

[h0x91b]

Applies progressive disclosure to the agent instructions: bulky reference material moves to where it is loaded on demand, while the load-bearing rules stay inline in AGENTS.md.

• New .claude/skills/verify-changes/SKILL.md — which vitest config covers what, what a change needs covered, house style (incl. the query patterns that flake under CI load), mocking Electrobun RPC and useT(), reproduce-under-load workflow for flakes, and coverage expectations.
• AGENTS.md Testing section reduced to the framework line, the commands block, a pointer to the skill, and the two hard gates. The gates and the mandatory browser QA / streamer-mode rules deliberately stay inline — an agent that never loads the skill must still hit them.
• Tip coolness rubric moved onto the TipScore type in src/mainview/tips.ts as a discriminated 1 | 2 | 3 | 4 | 5 union with the rubric in its doc comment; AGENTS.md now points at the type instead of restating 20 lines of prose. Side effect: an out-of-range score is now a type error.
• Key files list extended from 4 to 13 verified entries (shared types, RPC handler barrel, reducer, design tokens, keymap, shipped skill text, CLI exit codes, CI workflows, decision records) — the list exists so an agent knows these files are there, not as a summary of them.

Net: AGENTS.md 429 → 388 lines.