Harden pull_request_target workflows against untrusted PR input

[dhruvkb]

Fixes

Related to #5621 and #5622

Description

This PR hardens the two workflows that run on pull_request_target (and so have access to secrets while being triggered by untrusted PRs).

#5621 and #5622 tried to exploit pr_limit_reminders.yml by replacing count_user_reviewable_prs.js with a script that exfiltrates secrets. The attempt failed because actions/checkout checks out the base branch under pull_request_target, so the PR's code never ran (the run logs show the legitimate script's Slack username not found warning). However, a single ref: line would make the workflow exploitable, so this PR adds defence in depth:

pr_limit_reminders.yml

• Replaces the ACCESS_TOKEN PAT with the built-in GITHUB_TOKEN, scoped to contents: read and pull-requests: read. The script only reads open PRs and their labels in this public repo, so the PAT is unnecessary, and a future compromise would no longer leak it.
• Sets persist-credentials: false so the token is not written to .git/config.
• Sparse-checks out only automations/js/src, which is all the step needs.
• Adds a warning comment against checking out the PR head.
• Sets permissions: {} for the Slack job, which needs no token.

pr_ping.yml

• Sets permissions: {}, as the job makes no GitHub API calls.
• Escapes &, < and > in the PR title and author before interpolating them into Slack mrkdwn. Currently, a PR titled e.g. <!channel> pings the whole channel, and <https://example.com|text> renders a disguised link.

pr_automations.yml and pr_automations_init.yml were also reviewed and need no changes: the former already checks out main without credentials and validates the PR-built artifacts, and the latter runs on pull_request without secrets.

Testing Instructions

These workflows run from main on pull_request_target, so they can only be fully verified after merging. After merge:

1. Open a PR (or mark one ready for review) and confirm the "New PR notification" Slack message is unchanged, including for a title containing <, > or &.
2. Confirm the "Count user's open pull requests" job succeeds and logs the PR count for a maintainer with an entry in GH_SLACK_USERNAME_MAP. If the GraphQL query fails with a permissions error on GITHUB_TOKEN, that is the change to revisit.

Locally, just lint passes on the changed files (actionlint, prettier, codespell) and the escape helper was checked in Node against a malicious title.

Checklist

• My pull request has a descriptive title (not a vague title likeUpdate index.md).
• My pull request targets the default branch of the repository (main) or a parent feature branch.
• My commit messages follow best practices.
• My code follows the established code style of the repository.
• I added or updated tests for the changes I made (if applicable).
• I added or updated documentation (if applicable).
• I tried running the project locally and verified that there are no visible errors.
• I ran the DAG documentation generator (ov just catalog/generate-docs for catalog
  PRs) or the media properties generator (ov just catalog/generate-docs media-props
  for the catalog or ov just api/generate-docs for the API) where applicable.

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