feat(status): acknowledge a pull request the moment automatic commands start

Feature request

Give immediate feedback when PR-Agent starts working on a newly opened pull request.

Today an automatic command produces nothing observable until the model has answered:

• _perform_auto_commands_github sets config.is_auto_command, and every tool checks that flag before publishing its "Preparing …" progress comment — pr_reviewer.py, pr_description.py and pr_code_suggestions.py all suppress it for automatic runs;
• the suppression is deliberate (an automatic run should not add a throwaway comment to every PR), but it leaves no signal at all.

For the author this is indistinguishable from a webhook that never arrived, a misconfigured token, or a command that failed silently. On a large diff the wait is a minute or more.

Motivation

Proposed behaviour

Mark the PR head commit with a status as soon as the automatic commands start, and update it when they finish:

[config]
publish_run_status = false        # opt-in
run_status_context = "pr-agent"

• pending — "PR-Agent is running N command(s)" — published before the first command;
• success / failure — published after the last one.

A commit status rather than a comment, because it is not part of the conversation: it shows up in the PR's checks area immediately, is replaced in place on re-run, and adds no noise. Unlike the Checks API it needs no additional permission.

Providers without a status API (Bitbucket, Gerrit, CodeCommit, local) should be a silent no-op.