Add lint/format toolchain and split CI into concurrent jobs

[TheGreatAxios]

Summary

Stage 1 of CL-6802: port the workbench repo's CI/lint toolchain configuration. This PR lands the toolchain only — a separate stage 2 PR will apply the repo-wide mechanical fix batch (eslint --fix / prettier --write) after 5 currently-open PRs merge, to avoid conflicts.

• ESLint flat config (eslint.config.js): @eslint/js recommended + typescript-eslint strict + stylistic, zero-suppressions policy (noInlineConfig: true, reportUnusedDisableDirectives: "error"), underscore-prefixed args/vars allowed for no-unused-vars.
• Prettier: .prettierrc.json + .prettierignore (CHANGELOG.md excluded — formatting it would churn the whole file).
• package.json scripts: lint (prettier --check --cache . && eslint --cache .) and check (lint + typecheck + build + test, matching CI order).
• .github/workflows/ci.yml split into concurrent lint / typecheck / build-and-test jobs, each with a bun install cache keyed on bun.lock; the lint job also caches .eslintcache + prettier cache per-SHA with restore-keys. Concurrency group supersedes stale PR runs but never cancels main-branch commits. Existing corbits-specific steps (ripgrep install, randomized-order test run) stay in build-and-test.
• tsconfig.base.json: incremental: true. Typecheck scope unchanged (tests/evals inclusion is CL-5759).
• AGENTS.md: names bun run check as the single pre-PR gate.

The codebase is not yet lint-clean (1938 ESLint problems, 599 files with Prettier formatting issues found by an initial scan), so the CI lint job is continue-on-error: true for now with a TODO(CL-6802 stage 2) marker to flip it blocking once the mechanical fix batch lands. The lint script itself stays strict (non-zero exit on any violation).

No src/** code, vendor/, or Linear state touched.

Part of CL-6802
https://linear.app/abklabs/issue/CL-6802

Test plan

• bun run typecheck — clean
• bun run lint — executes, fails as expected (599 files need Prettier formatting, 1938 ESLint problems) — confirms the toolchain runs against real violations
• bun run check — chains lint → typecheck → build → test correctly, stops at lint (expected for stage 1)
• bunx yaml-lint .github/workflows/ci.yml — parses successfully

[linear-code]

CL-6802