[access-secret-dialog-cutover] - Remove modal secret dialog contracts

[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-dialog-cutover] - Remove modal secret dialog GUI implementation #1569.

Summary

Remove the fixed-list secret dialog input/result/host contracts after shared authorization rows own decisions.

Current Limitation / Root Cause

ISecretUseDialogHost and its input/result records encode the retired modal list workflow.

Affected Files

Path	Change
Phantom.Workspaces.Llm.Core/Secrets/ISecretUseDialogHost.cs	Remove or update only the named bounded production surface.
Phantom.Workspaces.Llm.Core/Secrets/SecretUseDialogInput.cs	Remove or update only the named bounded production surface.
Phantom.Workspaces.Llm.Core/Secrets/SecretUseDialogResult.cs	Remove or update only the named bounded production surface.
Focused test file	Add only the tests listed below.

Design / Fix

// Remove ISecretUseDialogHost.
// Remove SecretUseDialogInput and SecretUseDialogResult record families.

All authorization actions already flow through IUserAuthorizationRegistry before this commit.

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 contracts

Production types: ISecretUseDialogHost, SecretUseDialogInput, SecretUseDialogResult.

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

Expected Tests

Test Name	Class	What It Verifies
AccessMigration_ProductionTree_HasNoISecretUseDialogHost	AccessMigrationTests	Verifies the bounded cutover with no remaining production reference.
AccessMigration_ProductionTree_HasNoSecretUseDialogInput	AccessMigrationTests	Verifies the bounded cutover with no remaining production reference.
AccessMigration_ProductionTree_HasNoSecretUseDialogResult	AccessMigrationTests	Verifies the bounded cutover with no remaining production reference.