Document agentic contribution workflow

Author: dsyme

CONTRIBUTING.md

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+# Contributing to FSharp.Control.TaskSeq
+
+FSharp.Control.TaskSeq is primarily maintained through agentic development under human guidance.
+Maintainers discuss proposed work and then direct coding agents to make the complete change,
+including implementation, tests, documentation, and other affected files.
+
+## Start With an Issue
+
+We generally prefer contributions as [GitHub issues] rather than pull requests. Search for an
+existing report first. Issues may include proposed changes, patches, or links to forks or branches.
+Maintainers may refine the scope and assign the issue to an agent to implement and validate the
+complete change.
+
+## Repo Assist
+
+[Repo Assist] is an automated AI assistant that runs regularly in this repository. It may triage or
+respond to issues, investigate bugs, suggest improvements, and attempt implementations as draft pull
+requests. Its work is identified as automated and remains subject to human review; Repo Assist does
+not merge pull requests or make final maintenance decisions.
+
+Maintainers can invoke Repo Assist with `/repo-assist <instructions>` for a specific agentic task,
+such as investigating an issue, preparing a fix, adding tests, or updating documentation.
+
+## Pull Requests
+
+Every pull request must have a matching issue that has been discussed with the maintainers. Link the
+pull request to that issue and keep it focused. Maintainers may close a pull request and use the issue
+as the basis for an agent-produced implementation instead; the submitted analysis and code remain
+valuable inputs to that work.
+
+If a pull request is the agreed approach, follow [AGENTS.md], add tests and an `Unreleased` entry in
+`release-notes.txt` for user-visible changes, and run:
+
+```bash
+dotnet tool restore
+dotnet fantomas . --check
+dotnet build src/FSharp.Control.TaskSeq.sln -c Release
+dotnet test src/FSharp.Control.TaskSeq.Test/FSharp.Control.TaskSeq.Test.fsproj -c Release
+```
+
+[AGENTS.md]: AGENTS.md
+[GitHub issues]: https://github.com/fsprojects/FSharp.Control.TaskSeq/issues
+[Repo Assist]: https://github.com/githubnext/agentics/blob/main/docs/repo-assist.md