chore(agents): consolidate agent context and compress instruction files

Replace AGENT.md and .cursor/rules with a single AGENTS.md entry point plus
CODEMAP.md, and cut duplicated guidance out of the instruction files.

The same six non-negotiables were restated across seven files, so editing one
.tsx under packages/ loaded ~36.8KB of instructions to say them four times
over. Now ~28.7KB: generic React/Vitest material dropped, Fusion-specific
patterns kept.

Also corrects rules that were actively wrong:
- vite plugins were documented as `fusion-framework-vite-plugin-*`; every one
  of them is `@equinor/`-scoped
- the documented pre-PR gate `pnpm -w check` runs biome only, silently
  skipping fusion-lint; now `pnpm lint`
- documentation.instructions.md applied to `**/*.md`, firing its
  retrieval-corpus rules on changesets and agent files

Adds `pnpm verify:agent-context` plus a CI job so CODEMAP.md cannot drift from
the real workspace, and contributing/fusion-lint.md for lint knowledge that
previously lived only in agent memory.

No changeset: nothing under packages/* or cookbooks/* is touched.

Author: odinr