chore(ai): add repo-assist.md for automated repository assistant

[xperiandri]

Proposed Changes

Added a new .github/workflows/repo-assist.md workflow definition that introduces a repository automation assistant configuration and operating policy. The file defines scheduled daily and manual execution, checkout and .NET SDK setup, scoped permissions, and guarded output behaviors for comments, PR creation, issue updates, and label management. It also adds a detailed task playbook covering issue triage, bug-fix PRs, improvement PRs, dependency maintenance, stale PR nudges, release preparation, newcomer welcome flow, and monthly activity reporting. The workflow includes explicit quality and safety constraints such as mandatory build and test verification before PR creation, anti-spam rules, and required AI disclosure language.

Types of changes

What types of changes does your code introduce to FSharp.Control.R3?
Put an x in the boxes that apply

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your code.

• Build and tests pass locally (not verified from diff/commit evidence)
• I have added tests that prove my fix is effective or that my feature works (if appropriate)
• I have added necessary documentation (if appropriate)

Further comments

This is a large policy and automation addition concentrated in a single new file (390 lines) and affects repository process rather than library runtime behavior. It is intentionally additive and does not modify existing source, tests, or public API surface.

[Copilot]

Pull request overview

Adds an agentic “Repo Assist” workflow/policy document under .github/workflows/ to guide an automated repository assistant’s behavior (issue triage, draft PR creation, labeling, stale PR nudges, and monthly activity reporting) for the FSharp.Control.R3 repo.

Changes:

• Introduces a new Repo Assist workflow/policy definition with scheduled + manual triggering and guarded output behaviors (comments/PRs/issues/labels).
• Adds an operational playbook for issue triage, bugfix/improvement PR generation, dependency maintenance, release prep, and onboarding.
• Documents project-specific build/test/formatting constraints for assistant-generated changes.