The previous implementation POSTed Forgejo's run-page rerun web route,
which is CSRF-gated and answers a bare token POST with 404 — so the verb
never actually re-ran anything against the agent token.
Rework it to dispatch a fresh run of the workflow via the
GitHub-compatible workflow-dispatch API
(POST /repos/<o>/<r>/actions/workflows/<workflow>/dispatches {"ref":<branch>}),
which accepts a plain agent token (verified end-to-end on Forgejo 15.0.3).
A dispatched run is equivalent to the old empty-commit retrigger, minus
the commit.
The branch is resolved from exactly one of --pr (the PR head branch),
--run (branch + workflow looked up from that run in the Actions runs
list), or --branch (directly); --workflow picks the workflow file for
--pr/--branch (default ci.yml). Dispatch re-runs the whole workflow, so
the old --job single-job variant is dropped.
Also add workflow_dispatch to .forgejo/workflows/ci.yml for explicitness
(Forgejo 15.0.3 dispatches the pull_request workflow without it, but the
trigger makes the API path intent-clear and cross-version robust), remove
the now-unused Client::post_web_no_content, and update docs/tools/forge.md.
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