CI runs on every pull request regardless of target branch

## Summary

Drop the `branches: [main]` filter from the `pull_request` triggers so a PR
targeting any branch gets the full check set.

## Motivation

`ci.yml:6-7`, `pre-commit.yml:6-7`, and `links.yml:6-7` all filter
`pull_request` to `branches: [main]`. A PR that targets anything else — a
stacked PR on top of a feature branch — runs zero checks. The default-branch
ruleset only covers `~DEFAULT_BRANCH`, so nothing gates that merge either.

Exposure is bounded: whatever eventually reaches `main` goes through a PR that
is checked. The cost is that a stacked branch can accumulate unverified commits,
and the failure surfaces late and all at once. alunduil-chezmoi leaves its
`pull_request` triggers unfiltered.

Low severity, one line per workflow.

## Scope

- Remove `branches: [main]` from the `pull_request:` trigger in `ci.yml`,
`pre-commit.yml`, and `links.yml`.
- Leave the `push:` triggers filtered to `main`, so a branch push with an open
PR still produces one run rather than two.

## Acceptance criteria

- [ ] A PR targeting a non-`main` branch runs the build matrix, `Hooks`, and
`Check links`.
- [ ] A push to a branch with an open PR produces one set of runs, not two.

## Additional context

Surfaced by a CI and checks audit.

Start by inspecting the pull_request and push triggers at lines 6-7 of ci.yml, pre-commit.yml, and links.yml. Remove only the pull_request branch filters, preserve the push filters, and verify that non-main pull requests receive the build matrix, Hooks, and Check links while a branch push with an open pull request does not create duplicate runs.

索引モデルが issue の本文から書いたものです。