fix(ci): install declared runtimes

[nerdalytics]

What changed

• Generate an ephemeral .tool-versions from the tracked package.json
  runtime declarations before mise-action runs.
• Fail early if packageManager and engines.pnpm disagree.
• Refresh the README with the current package layout, Node 26.5.0, pnpm
  11.16.0, local mise.local.toml setup, available tasks, and the
  authoritative mise run ci workflow.

Why

The first trunk build enabled by #90 failed before dependency installation.
CI had no tracked mise tool declarations because mise.local.toml is
intentionally ignored, so pnpm store path --silent failed with
pnpm: command not found. That left the cache path empty and caused
actions/cache to stop the job.

Generating .tool-versions before setup lets the pinned mise action install
the exact runtimes declared by the project without committing developer-local
tool configuration.

Impact

GitHub Actions can install Node and pnpm before resolving the pnpm store, while
local contributors retain ignored per-checkout mise configuration. Runtime
version drift between the pnpm engine and package-manager declaration now
fails during setup.

Validation

• Reproduced the original failure in GitHub Actions run 30111620541.
• Verified the generated tool declarations are exactly node 26.5.0 and
  pnpm 11.16.0.
• Ran git diff --check successfully.
• Ran the full mise run ci pipeline successfully.
• GitHub Actions Build and Test passed in run 30114935725.