docs: create CI.md to document e2e pipeline design and decisions

Our CI pipeline has grown complex recently with the addition of smoke/nightly test split, changed-file inference suite router (two-list approach), and spec map coverage check. Suggested by @jasonmadigan in PR review.

What to document

• General setup — how the e2e pipeline works end to end (oinc cluster, plugin dev server, Playwright)
• Test tags — @smoke vs @nightly: what they mean, when to use each
• Suite router — two-list design (specs vs test_specs), fallback behaviour, component mapping
• Spec map check — what build/check-spec-map.sh does, how to run locally (yarn check:spec-map)
• Design decisions — why two lists instead of one flag, why nightly runs on test file changes only
• Adding new tests — checklist: tag, update suite-router mapping, run check:spec-map

Location

docs/ci.md — referenced from CLAUDE.md and AGENTS.md