Stop pretending include-checks works, and point it at .oasdiff.yaml

[reuvenharrison]

Closes #196.

The bug

oasdiff retired the optional-checks mechanism include-checks drove, and now ignores the flag:

"include-checks": "it is now ignored; use --severity-levels to enforce a check as breaking"

The actions still advertised it as functional. So the documented way to enforce a check did nothing at all: the user believes a check is enforced, no error appears, and it never fires. Silence is the worst of the available failure modes, and it is the whole of the bug.

The fix

The action no longer forwards the flag, and emits a warning annotation instead:

::warning::oasdiff: 'include-checks' is ignored, the optional-checks mechanism it drove was
retired. To make a check fail the build, add 'severity-levels: <file>' to .oasdiff.yaml,
listing the check id with level 'err', and set 'fail-on: ERR'.

The input itself stays, so workflows that pass it today keep working rather than failing on an unknown input. Forwarding it was pointless (oasdiff ignores it) and would break outright if the flag is ever removed rather than merely ignored.

Why there is no severity-levels input

#196 proposed adding one. It is deliberately not here.

.oasdiff.yaml is the configuration surface these actions converge on, and the inputs are a deliberate subset, not an aspiration to cover the flag set. Counting breaking:

	count	examples
oasdiff flags with an input	10	fail-on, composed, err-ignore
oasdiff flags without	20	auto-upgrade, stability-level, match-path, prefix-base, format, lang, template

So not having an input is the norm. severity-levels already worked through the config file before this PR, needing no code at all, and adding an input would grow a surface we intend to stop growing. It would also add another positional argument, in a scheme where a mis-ordered index corrupts a run silently rather than erroring.

The deprecation message and the README therefore teach the config file:

# .oasdiff.yaml
severity-levels: ./oasdiff-levels.txt
fail-on: ERR

# oasdiff-levels.txt
api-version-not-bumped          err
api-major-version-not-bumped    err

That is the recipe #154 needed.

Tests

• severity-levels via .oasdiff.yaml — downgrades api-removed-without-deprecation (this repo's only breaking finding on the test specs) to info, and asserts fail-on: ERR stops failing. Sits alongside the existing yaml-config jobs for fail-on and err-ignore, so the config path is pinned rather than assumed.
• include-checks — passes not-a-real-check and asserts the run is unaffected. This fails if the action ever forwards the input again, since oasdiff would reject the name.

Also verified by running the entrypoint against a stubbed oasdiff that logs its argv: neither --include-checks nor --severity-levels reaches the CLI, and the annotation is emitted.