Apply pre-commit formatting and enforce it in CI

[mcocdawc]

eckit has a .pre-commit-config.yaml, but nothing in CI ever runs it — neither
.github/workflows/ci.yml nor .ci/manifest.toml invokes pre-commit. So the config is
advisory only, and the tree has drifted away from it.

Two things to do

1. Apply the hooks once. Running pre-commit run --all-files today rewrites ~19 files:
import reordering (docs/conf.py), trailing whitespace (src/eckit/sql/sqll.l,
sqly.y, SchemaAnalyzer.cc), missing/extra final newlines (tests/parser/*.yaml),
plus python/, src/sandbox/ and tests/ files. Best landed as its own
formatting-only commit so it stays reviewable and doesn't mix with behaviour changes.

2. Fix yamllint, then enforce in CI. yamllint runs with --strict, so warnings are
errors, and it currently fails on GitHub workflow files:

.github/workflows/ci.yml
  1:1  warning  missing document start "---"  (document-start)
  3:1  warning  truthy value should be one of [false, true]  (truthy)

The truthy one is inherent to workflow YAML — GitHub requires a bare on: key, which
yamllint reads as the boolean true. Quoting it as "on": would satisfy the linter but
edits the trigger block, so excluding .github/workflows/ from yamllint (or relaxing
those two rules there) is probably the better fix.

This is not hypothetical: it currently blocks any commit that touches a workflow file,
because pre-commit runs the hook on staged files. It blocked a two-line change to
ci.yml in this repo today, which had to go in with --no-verify.

Once green, add a pre-commit job to ci.yml. There is a shared action for it:

  pre-commit:
    runs-on: ubuntu-slim
    env:
      FORCE_JAVASCRIPT_ACTIONS_TO_NODE24: 'true'
    steps:
      - uses: actions/checkout@v6
      - uses: ecmwf/ci-infrastructure/actions/pre-commit@main

ecflow, stack-dependencies and ci-infrastructure already use it.

Also worth a look

.pre-commit-config.yaml lists pre-commit/pre-commit-hooks twice, at two different revs
(v5.0.0 and v4.5.0), which looks like accidental drift — the other repos in the rollout
are on v6.0.0.