docs: salvage F# agent and language guidance

rules/fsharp/coding-style.md

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+---
+paths:
+  - "**/*.fs"
+  - "**/*.fsx"
+---
+# F# Coding Style
+
+> This file extends [common/coding-style.md](../common/coding-style.md) with F#-specific content.
+
+## Standards
+
+- Follow standard F# conventions and leverage the type system for correctness
+- Prefer immutability by default; use `mutable` only when justified by performance
+- Keep modules focused and cohesive
+
+## Types and Models
+
+- Prefer discriminated unions for domain modeling over class hierarchies
+- Use records for data with named fields
+- Use single-case unions for type-safe wrappers around primitives
+- Avoid classes unless interop or mutable state requires them
+
+```fsharp
+type EmailAddress = EmailAddress of string
+
+type OrderStatus =
+    | Pending
+    | Confirmed of confirmedAt: DateTimeOffset
+    | Shipped of trackingNumber: string
+    | Cancelled of reason: string
+
+type Order =
+    { Id: Guid
+      CustomerId: string
+      Status: OrderStatus
+      Items: OrderItem list }
+```
+
+## Immutability
+
+- Records are immutable by default; use `with` expressions for updates
+- Prefer `list`, `map`, `set` over mutable collections
+- Avoid `ref` cells and mutable fields in domain logic
+
+```fsharp
+let rename (profile: UserProfile) newName =
+    { profile with Name = newName }
+```
+
+## Function Style
+
+- Prefer small, composable functions over large methods
+- Use the pipe operator `|>` to build readable data pipelines
+- Prefer pattern matching over if/else chains
+- Use `Option` instead of null; use `Result` for operations that can fail
+
+```fsharp
+let processOrder order =
+    order
+    |> validateItems
+    |> Result.bind calculateTotal
+    |> Result.map applyDiscount
+    |> Result.mapError OrderError
+```
+
+## Async and Error Handling
+
+- Use `task { }` for interop with .NET async APIs
+- Use `async { }` for F#-native async workflows
+- Propagate `CancellationToken` through public async APIs
+- Prefer `Result` and railway-oriented programming over exceptions for expected failures
+
+```fsharp
+let loadOrderAsync (orderId: Guid) (ct: CancellationToken) =
+    task {
+        let! order = repository.FindAsync(orderId, ct)
+        return
+            order
+            |> Option.defaultWith (fun () ->
+                failwith $"Order {orderId} was not found.")
+    }
+```
+
+## Formatting
+
+- Use `fantomas` for automatic formatting
+- Prefer significant whitespace; avoid unnecessary parentheses
+- Remove unused `open` declarations
+
+### Open Declaration Order
+
+Group `open` statements into four sections separated by a blank line, each section sorted lexically within itself:
+
+1. `System.*`
+2. `Microsoft.*`
+3. Third-party namespaces
+4. First-party / project namespaces
+
+```fsharp
+open System
+open System.Collections.Generic
+open System.Threading.Tasks
+
+open Microsoft.AspNetCore.Http
+open Microsoft.Extensions.Logging
+
+open FsCheck.Xunit
+open Swensen.Unquote
+
+open MyApp.Domain
+open MyApp.Infrastructure
+```