Align lint/format/test config with the typescript-action template

[SVAGEN26]

Closes #465.

Starting point

The dependency half of #465 is already done — dependabot moved the repo onto the
ESM-only majors (@actions/core 3, @actions/exec 3, @actions/tool-cache 4,
@actions/cache 6), and "type": "module" + rollup + flat ESLint config are all
in place. What never caught up is the surrounding config — and most of it turned
out not to be exercised at all.

What was actually wrong

__tests__ was never linted. lint was eslint src/**/*.ts, so the
eslint-plugin-jest setup and the tsconfig.test.json entry in
parserOptions.project were dead weight. Now eslint . covers the tree. That
needs the root config files to belong to a project, so parserOptions.project
is replaced by the template's projectService + allowDefaultProject, and
coverage/ joins dist/ in the ignores.

eslint.config.mjs had never been checked by its own rules. Once linted it
reported an unused js import and an unused __filename/__dirname pair left
over from the flat-config migration. Removed, along with the node:path and
node:url imports that only fed them.

rollup.config.ts imported nodeResolve as a default import, which
import/no-named-as-default flags as soon as the file is linted. Switched to the
named import.

Jest config was doubled up. preset was the deprecated
ts-jest/presets/default-esm while transform already passed useESM: true,
and moduleNameMapper re-implemented what ts-jest-resolver was already doing.
Reduced to the template's preset: 'ts-jest' + resolver, plus its reporters.

tsconfig now excludes __tests__ and dist as the template does.
tsconfig.test.json declares its own exclude, so tests are still type-checked
there — tsc --noEmit -p tsconfig.test.json passes.

Nothing enforced any of it. test.yml ran only package and test, never
lint or format, so all of the above could drift indefinitely. Both are now steps
in the build job.

Scripts move to the template's format:write / format:check names and run over
the whole tree instead of **/*.ts.

Two commits

1. the config change
2. the one-time prettier pass over the Markdown and YAML that format:check now
   covers — kept separate because it is 118 lines of pure churn. git diff -w
   reduces it to prettier's own singleQuote preference and Markdown table
   padding; no content changes.

Verification

• npm run package produces a byte-identical dist/index.js — git status dist/ is clean, so check-dist is unaffected and the shipped action does not
  change.
• npm run lint, npm run format:check and tsc --noEmit -p tsconfig.test.json
  all pass.
• npm test reports the same results as before the change (the failures are
  environmental — the suite really does apt install omc — and are identical on
  main).

Deliberately not done

eslint-plugin-github and @stylistic are kept rather than swapped for the
template's prettier-plugin-based ESLint config. The issue asks for the ESLint
configuration to resemble the template, but that swap changes which rules apply
to src/, which is a behavioural change to the action source rather than config
alignment. Happy to do it as a follow-up if you want the full template ruleset.

Also noticed while running the suite: the tests leave installLibs.mos and
linux-64.tar.gz in the working tree and neither is gitignored. Left alone here.

🤖 Generated with Claude Code

[SVAGEN26]

@AnHeuermann review please — this closes #465, which you assigned to @JKRT.

(Claude Code agent acting on behalf of @JKRT.)

Two things worth knowing before you look:

1. The dependency half of the issue was already done. Dependabot had
   already moved us to the ESM-only majors (@actions/core 3, exec 3,
   tool-cache 4, cache 6). What was left was the config around them — and
   most of it turned out not to be exercised: lint only ever covered src,
   so __tests__ was never linted, and CI ran neither lint nor format, so
   nothing enforced any of it.

2. CI has not run. All four workflows are sitting at action_required
   because SVAGEN26 is a first-time contributor to this repo. If you approve
   the runs, build-test and check-dist should both go green — locally
   npm run package produces a byte-identical dist/index.js, so the shipped
   action is unchanged.

The second commit is a one-time prettier pass over the Markdown and YAML that
format:check now covers. It looks big (118 lines) but git diff -w reduces it
to quote style and Markdown table padding — no content changes. Kept separate so
the config commit stays readable.

One thing I did not do, deliberately: the issue asks for the ESLint config to
match the template, but that would mean dropping eslint-plugin-github and
@stylistic for the template's prettier-plugin setup. That changes which rules
apply to src/, which is a behavioural change to the action source rather than
config alignment, so I left it. Say the word if you want the full template
ruleset and I will do it as a follow-up.

[AnHeuermann]

LGTM