[access-secret-dialog-cutover] - Remove modal secret dialog GUI implementation

[JoshuaRowePhantom]

Part of #1512 and master #1505

Parent

• Direct parent: [access-secrets-github] - Track stored-secret environment and GitHub executor slices #1512.
• Architecture master: [access-broker] - Unify credential-use authorization, acquisition, renewal, and presentation #1505.

Dependencies

• Depends on: [access-secret-materialization] - Bind manifest secret handles to broker leases #1540.
• Depends on: [access-authorization-loading-ui] - Render authorization grid in LoadingWindow #1552.

Summary

Remove the obsolete Avalonia modal host, view model, and window as one GUI-only cutover.

Current Limitation / Root Cause

The modal GUI remains a bypassing fixed batch surface after its contracts are removed.

Affected Files

Path	Change
Phantom.Workspaces/Services/Secrets/AvaloniaSecretUseDialogHost.cs	Remove or update only the named bounded production surface.
Phantom.Workspaces/ViewModels/SecretUseDialogViewModel.cs	Remove or update only the named bounded production surface.
Phantom.Workspaces/SecretUseDialogWindow.axaml	Remove or update only the named bounded production surface.
Focused test file	Add only the tests listed below.

Design / Fix

// Remove AvaloniaSecretUseDialogHost.
// Remove SecretUseDialogViewModel.
// Remove SecretUseDialogWindow and its code-behind.

Shared AuthorizationRequestRowsControl remains the only credential-use decision UI.

Security and Privacy Invariants

• No credential material, provider artifact, unsafe identifier, or raw provider output is introduced or copied during removal.
• Replacement callers already use typed broker requests and caller-owned leases.
• Removal fails the build if an undeclared production reference remains; it does not leave a fallback.

Exclusions

• No unrelated provider, UI, persistence-format, or behavioral redesign.
• No bridge, migration reader, or dual production path.

Commit Plan

Commit 1 — [access-secret-dialog-cutover] - Remove modal secret dialog GUI implementation

Production types: AvaloniaSecretUseDialogHost, SecretUseDialogViewModel, SecretUseDialogWindow.

Acceptance: one independently buildable commit affecting only the files above plus AccessMigrationTests.

Expected Tests

Test Name	Class	What It Verifies
AccessMigration_GuiTree_HasNoAvaloniaSecretUseDialogHost	AccessMigrationTests	Verifies the bounded cutover with no remaining production reference.
AccessMigration_GuiTree_HasNoSecretUseDialogViewModel	AccessMigrationTests	Verifies the bounded cutover with no remaining production reference.
AccessMigration_GuiTree_HasNoSecretUseDialogWindow	AccessMigrationTests	Verifies the bounded cutover with no remaining production reference.