feat: add action.yml

[doc-han]

What this does

Turns this repo into something Lightning and kit can call directly from their
own CI, so a breaking change on either side shows up on the PR that
introduced it — not just on a manual dispatch here
(lightning#4784's original
ask). action.yml is a composite action wrapping the same
prep-and-run sequence the dispatch workflow already did: resolve the specs,
install the toolchain the Lightning checkout pins, boot the pair, run the
suite, upload logs.

The dispatch workflow (test-lightning-branch.yml)
now calls that action (uses: ./) instead of duplicating its steps, so it
doubles as the action's own test.

How to use and test it

bun run typecheck
bun src/cli.ts resolve --lightning ../lightning --worker ../kit   # prints {lightning, worker} JSON

Not yet run through a real uses: OpenFn/kit-lightning-integration@... call
from another repo — that needs a push and a tag (see reviewer guide). Checked
so far: both YAML files parse, actionlint reports no findings on the
workflow, and resolve produces clean JSON on stdout for a local path, a
bare ref (triggers a clone), and the owner/repo#ref form the workflow
itself now passes.

Where to look, in order

1. action.yml — start here; the header comment is the spec
   (the two caller snippets, the two requirements: caller provides Postgres,
   Linux runner).
2. src/cli.ts — new resolve subcommand: turns a spec into
   a checkout (cloning if needed) without booting anything, prints
   {lightning, worker} as JSON. This is the seam the action needed — it has
   to know where .tool-versions lives before it can install the
   toolchain, which the old workflow only solved by hardcoding
   actions/checkout into .cache/lightning itself.
3. src/source.ts — one-line change: the fetch-progress
   log moved to stderr, so resolve's stdout stays clean JSON for the
   action's jq to parse.
4. test-lightning-branch.yml
   — collapsed to checkout + Postgres service + uses: ./, passing
   owner/repo#ref for the action to resolve itself.
5. README.md — new "Using it from another repo" section with
   the caller snippet and the @v1-not-@main pinning note.