chore: prepare for v2.0.0 (#405)

## Summary

This PR prepares `supabase/setup-cli` for `v2.0.0`.

The main goal of this release is to simplify the action and modernize
the repo/tooling around a Bun-based implementation, while tightening
workflows, tests, and documentation.

## What Changed

### Action runtime
- switched the action from a Node/compiled `dist` runtime to a Bun-based
composite action
- removed the checked-in `dist/` output entirely
- simplified the action source down to a single runtime file in
`src/main.ts`
- kept the public action interface the same:
  - `with.version`
  - `outputs.version`

### Tooling
- switched package management and local tooling from npm to Bun
- removed Rollup and the build step
- replaced Jest with Bun’s native test runner
- replaced Prettier with `oxfmt`
- replaced ESLint with `oxlint`
- enabled type-aware/type-check linting with `oxlint-tsgolint`
- simplified TypeScript config to a single `tsconfig.json` extending
`@tsconfig/bun`

### Tests
- moved tests next to the runtime source
- rewrote tests to focus on meaningful user-facing action behavior
- added coverage for:
  - default entrypoint execution
  - latest version installs
  - legacy version installs
  - modern pinned version installs
  - failure when the installed CLI cannot report a version
- action code coverage is now `100%`

### Workflows
- renamed workflow files for clarity:
  - `test.yml` -> `ci.yml`
  - `start.yml` -> `e2e.yml`
- updated workflow/job naming so required checks are clean and stable:
  - `CI`
  - `E2E`
  - `CodeQL`
  - `Licensed`
- added aggregate PR-facing checks so branch protection does not need
matrix legs
- made CI and E2E skip heavy jobs on draft PRs
- made E2E run automatically on ready PRs and new commits
- simplified CodeQL config by removing the separate config file
- updated action pins to current releases using commit SHAs
- refined Dependabot for Bun-era updates and non-major auto-merge

### Docs
- refreshed `README.md` and `docs/index.md` for the new v2 behavior
- updated examples to use `@v2`
- added a practical example for exporting local Supabase env vars after
`supabase start`
- removed stale references to old local/dev flows

## Breaking / Notable Changes

- the action now runs as a Bun-based composite action instead of a
prebuilt JavaScript action
- no checked-in `dist/` artifacts anymore
- self-hosted runners now need the prerequisites expected by the
composite action path:
  - `bash`
- network access to install Bun/dependencies and download the Supabase
CLI

## Validation

Verified locally with:
- `bun run format:check`
- `bun run lint`
- `bun test`
- `bun run ci`

Also updated workflows and branch-protection-friendly check names so PR
validation is cleaner going forward.

## Follow-up

After merge, branch protection should require only:
- `CI`
- `E2E`
- `CodeQL`
- `Licensed`

---------

Co-authored-by: licensed-ci <licensed-ci@users.noreply.github.com>

.github/workflows/e2e.yml

@@ -0,0 +1,71 @@
+name: E2E
+on:
+  pull_request:
+    types:
+      - opened
+      - synchronize
+      - reopened
+      - ready_for_review
+      - converted_to_draft
+  push:
+    branches:
+      - main
+    tags:
+      - "v[0-9]+.[0-9]+.[0-9]+"
+  schedule:
+    # * is a special character in YAML so you have to quote this string
+    - cron: "30 1,9 * * *"
+  workflow_dispatch:
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+defaults:
+  run:
+    shell: bash
+
+permissions:
+  contents: read
+
+jobs:
+  e2e: # make sure the action works on a clean machine without building
+    if: ${{ github.event_name != 'pull_request' || !github.event.pull_request.draft }}
+    runs-on: ubuntu-latest
+    timeout-minutes: 45
+    strategy:
+      fail-fast: false
+      matrix:
+        version:
+          - 1.178.2
+          - 2.33.0
+          - latest
+        pg_major:
+          - 14
+          - 15
+          - 17
+        exclude:
+          - version: 1.178.2
+            pg_major: 17
+    steps:
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+        with:
+          persist-credentials: false
+      - uses: ./
+        with:
+          version: ${{ matrix.version }}
+      - run: supabase init
+      - run: |
+          sed -i -E "s|^(major_version) .*|\\1 = ${{ matrix.pg_major }}|" supabase/config.toml
+      - run: supabase start
+
+  e2e-check:
+    if: ${{ always() && github.event_name == 'pull_request' }}
+    name: E2E
+    runs-on: ubuntu-latest
+    needs: [e2e]
+    timeout-minutes: 5
+    steps:
+      - run: |
+          e2e_result="${{ needs.e2e.result }}"
+          [[ "$e2e_result" == "success" || "$e2e_result" == "skipped" ]]