Default branch is renamed from master to main

## Summary

Update every woodland-generators config that pins `master` so the repo
aligns with the post-rename default of `main`. The rename itself and the
branch-protection re-apply are infra-side (alunduil/alunduil-infrastructure#24).

## Motivation

`main` is the GitHub default for new repos and what most upstream docs,
action examples, and Renovate presets assume. The repo predates the
change and still tracks `master`. Aligning removes a small ongoing tax
(every doc snippet, every preset that targets `main` needs translation)
and one less surprise for contributors.

Surfaced during #184 / PR #257 review.

## Scope

- Workflow `branches:` lists in `.github/workflows/{ci,bench,pre-commit,release-please}.yml`
(3 push/pull_request entries plus the `refs/heads/master` concurrency
guards in each).
- `renovate.json`: `baseBranchPatterns: ["master"]` → `["main"]`.
- Update local clones: `git branch -m master main && git fetch origin && git branch -u origin/main main && git remote set-head origin -a`.

`release-please-config.json` and `.release-please-manifest.json` don't
pin a branch name — no change needed there.

Handled in alunduil/alunduil-infrastructure#24 (out of scope here):

- The actual rename (`gh api .../rename-branch`).
- Terraform-side `default_branch = "main"` flip in the `woodland_generators` module call.
- Branch protection re-apply on `main` (follows automatically once the default moves).

## Acceptance criteria

- [ ] All workflows trigger on `main` and the bench/CI/pre-commit
concurrency guards no longer reference `refs/heads/master`.
- [ ] Renovate's next dashboard refresh targets `main` (visible on
Dependency Dashboard #150 or the next bump PR's base ref).
- [ ] `grep -rln 'master' --exclude-dir=node_modules --exclude-dir=.git --exclude=CHANGELOG.md`
returns no project-config hits (`.vale/styles/...` matches are
dictionary entries — expected).

## Additional context

- Open PRs at the time of the rename keep their old base ref; GitHub
auto-redirects but the PR's UI may need a re-target. Land this when
the PR queue is small.
- Blocked by the rename in alunduil/alunduil-infrastructure#24: these
triggers target `main`, so `main` has to exist first — merging them
onto a still-`master` default would just stop CI from firing. Land
this once the rename has flipped the default branch.