feat: replace fork IT trigger comment with pull_request_target approval gate

[stephanme]

Fork PRs now automatically trigger integration test workflows on each push. A lightweight gate job pauses execution pending approval by a committer via the fork-pr-integration-test GitHub environment. Same-repo PRs skip the gate and run tests immediately.

Removes the fork-integration-trigger.yml workflow and report-fork-status action, both of which were only needed for the old dispatch-based approach.

[stephanme]

Proposal by @FloThinksPi

Makes things simpler using standard Github features (pull_request_target trigger + environment gate): no magic comment that you have to remember, no extra status reporting with duplicated checks (skipped and green ones).

[stephanme]

ITs don't run because there is a bootstrapping problem. Here's what happens with this PR open:

• pull_request event fires → GitHub looks for pull_request trigger in the HEAD branch's workflow files → the HEAD branch already has pull_request_target (not pull_request) → no match, ITs don't run
• pull_request_target event fires → GitHub always looks at the BASE branch (main) for the workflow files → main still has the old pull_request trigger → no match, ITs don't run

Neither trigger fires because this PR is changing the trigger itself. Once it merges to main, future PRs will work correctly.

To run ITs for this PR's changes, use workflow_dispatch to manually trigger each IT workflow from the fork-pr-validation branch. But this won't update the required status checks.

To handle this, a repo admin can temporarily allow admins to bypass the branch protection rules and then merge the PR.

[stephanme]

Ran manually the DAV Integration Tests.
They succeeded and the gate was ignored (no fork PR).

[stephanme]

Merged bypassing the missing ITs (see comment above).