feat: expose release-please outputs to calling workflows

[TheMeinerLP]

release-please.yml runs the action but forwards none of its outputs, so a caller cannot tell whether a release was actually cut.

Why this matters

The README documents this pattern in two places, and it has never worked:

publish:
  needs: release-please
  if: needs.release-please.outputs.release_created == 'true'

needs.release-please.outputs.release_created resolves to an empty string, the condition is false, and the follow-up job is skipped. The release gets tagged and nothing is published — a failure that surfaces only at the next release, and looks like a publishing problem rather than a wiring one. The Docker example at line ~135 has the same issue with outputs.version.

What this adds

release_created, releases_created, tag_name, version, sha, paths_released and prs.

The names are verified against the action's source, not assumed: action.yml declares no outputs: section at all — they are set dynamically through core.setOutput() in src/index.ts. Two things fall out of that:

• tagName is deliberately published as tag_name (and notes as body, url as html_url) for backwards compatibility.
• Per-package outputs on a multi-package manifest are prefixed with the package path (<path>--release_created). Those names are not knowable ahead of time, so a reusable workflow cannot declare them. That is documented on release_created, pointing at releases_created and paths_released instead.

Also in here

The README's release-please example now shows the gating pattern it was already assuming, plus an output table. It was pinned to @v2 — the major alias the pin strategy says is deliberately not maintained — so it now points at a release tag.

Verification

YAML validated. The output names are cross-checked against src/index.ts@v5:

setPathOutput(path, 'release_created', true)
if (key === 'tagName') key = 'tag_name'
core.setOutput('releases_created', ...)
core.setOutput('paths_released', ...)
core.setOutput('prs', ...)

Backwards compatible — this only adds outputs, no input or behaviour changes. Existing callers are unaffected.