docs: update AGENTS.md test target from net6.0 to net9.0

The test project was updated to target net9.0 but AGENTS.md still
referenced net6.0 in two places. Update to reflect actual state.
Also note that FSharp.Core is pinned to 6.0.1 intentionally, to
ensure compatibility with the minimum supported version.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: github-actions[bot]

AGENTS.md

@@ -7,7 +7,7 @@ FSharp.Control.TaskSeq is an F# library providing a `taskSeq` computation expres
 ## Repository Layout
 
 - `src/FSharp.Control.TaskSeq/` — Main library (netstandard2.1)
-- `src/FSharp.Control.TaskSeq.Test/` — xUnit test project (net6.0)
+- `src/FSharp.Control.TaskSeq.Test/` — xUnit test project (net9.0)
 - `src/FSharp.Control.TaskSeq.SmokeTests/` — Smoke/integration tests
 - `src/FSharp.Control.TaskSeq.sln` — Solution file
 - `Version.props` — Single source of truth for the package version
@@ -97,7 +97,7 @@ All workflows are in `.github/workflows/`:
 - F# source files use `.fs` extension; signature files use `.fsi`.
 - `TreatWarningsAsErrors` is enabled for all projects.
 - File ordering matters in F# — the `<Compile>` order in `.fsproj` files defines compilation order.
-- The library targets `netstandard2.1`; tests target `net6.0` with `FSharp.Core` pinned to `6.0.1`.
+- The library targets `netstandard2.1`; tests target `net9.0` with `FSharp.Core` pinned to `6.0.1` (intentional: ensures compatibility with the minimum supported version).
 - NuGet packages are output to the `packages/` directory.
 
 ## Release Notes