fix: resolve push_files commit action per file - #643

What

createCommit built its payload with action: "create" hardcoded for every entry, so
push_files could only ever add files. GitLab's Commits API rejects create for a
path already tracked in the branch:

400 A file with this name already exists

The action is now resolved per file: update when the path exists in the target branch,
create when it does not.

const resolvedActions = await Promise.all(
  actions.map(async action => ({
    action: (await repositoryFileExists(projectId, action.path, branch)) ? "update" : "create",
    ...
  }))
);

repositoryFileExists issues a HEAD against the Repository Files API — headers rather
than the whole blob, since a commit may carry several files and their current content is
irrelevant to the decision. A branch that does not exist yet answers 404 exactly like a
missing file, which is the right answer: a commit that also creates the branch has to use
create.

create_or_update_file already does the same existence probe for a single file, so this
brings push_files in line with it rather than introducing a new pattern.

Why

Any commit touching existing code failed, which makes push_files unusable for the most
common editing workflow — an agent reading a file and writing it back. The error also
reads like a GitLab problem rather than a missing capability, so it is easy to
misdiagnose as a permissions or path issue.

The only working way to change a tracked file today is create_or_update_file, which
handles one file per commit — so a multi-file change cannot be made atomic at all.

No existing issue; happy to open one if you'd prefer to track it separately.

How tested

• npm run test:mock — 565 passing, including the new suite.
• npm run test:consumer-smoke — passes.
• npx tsc --noEmit, npm run build, npm run check:runtime-deps,
  npm run check:skill-sync — all clean.
• npx tsx scripts/generate-tool-docs.ts — regenerated; docs/tools/ committed with no
  drift.

New file test/test-push-files.ts (mock GitLab server, following the
test-create-repository.ts pattern) covers three cases:

• a path that already exists in the branch → action: "update"
• a path that does not → action: "create"
• a commit mixing both → resolved per file

The first case fails against main, where every action is create.

MockGitLabServer.addMockHandler gained "head" in its method union so the probe can be
stubbed. The dispatcher already keyed on the real request method, so nothing changed at
runtime.

Not run locally: npm run test:live (needs a real GitLab token) and
mkdocs build --strict (no local MkDocs). The docs change is a single generated
description line with no new links.

Breaking changes

None. No schema change, no new parameter. The only behavioural difference is that a call
which previously failed with a 400 now succeeds; callers that only ever pushed new files
are unaffected.

The tool description changes from "Push multiple files in a single commit" to "Create or
update multiple files in a single commit", and docs/tools/ is regenerated to match.