[access-host-threading] - Thread one broker through all production composition paths

[JoshuaRowePhantom]

Part of #1507 and master #1505

Parent

• Direct parent: [access-host] - Track startup composition handoff and lifecycle slices #1507.
• Architecture master: [access-broker] - Unify credential-use authorization, acquisition, renewal, and presentation #1505.

Dependencies

• Depends on: [access-host] - Compose one AccessHostServices graph #1561.

Summary

Replace remaining service-bag and factory construction inputs with the one host broker.

Current Limitation / Root Cause

Root/resumed/subagent sessions and repository, usage, MCP, Dev Tunnel, and VS Code composition can receive different acquisition dependencies.

Affected Files

Path	Change
Phantom.Workspaces/Services/AgentServicesComposition.cs	Implement the bounded production change owned by this issue.
Phantom.Workspaces/Services/Access/AccessBrokerProvider.cs	Implement the bounded production change owned by this issue.
Phantom.Workspaces/ViewModels/MainWindowViewModel.cs	Implement the bounded production change owned by this issue.
Focused test file	Add only the tests listed below.

Design / Fix

This slice implements the following exact contract:

public sealed record HostAgentServicesOptions
{
    public required IAccessBroker AccessBroker { get; init; }
}

All launch and connection paths receive the exact same broker reference. No provider is constructed in a leaf composition site.

Use required-init properties and named object initializers. The commit must build and its focused tests must pass without relying on a second implementation slice landing in the same commit.

Security and Privacy Invariants

• Release/use authorization occurs before material access or provider interaction whenever this slice can reach either boundary.
• No token, secret value, OAuth state, verifier, authorization URL, signed URL, device code, callback, account name, unsafe identifier, or credential-bearing result enters UI-safe state, logs, notifications, entities, or persistence.
• Typed results remain caller-owned and disposable; provider/cache artifacts remain executor-owned.
• Errors are typed and sanitized; no broad catch or success-shaped fallback is introduced.

Exclusions

• No batch API, compatibility bridge, legacy-data migration, silent interactive renewal, or unrelated provider work.
• No production types beyond the bounded files/classes named here.
• No UI work unless this issue is explicitly a UI slice.

Commit Plan

Commit 1 — [access-host-threading] - Thread one broker through all production composition paths

Production scope: Phantom.Workspaces/Services/AgentServicesComposition.cs, Phantom.Workspaces/Services/Access/AccessBrokerProvider.cs, Phantom.Workspaces/ViewModels/MainWindowViewModel.cs.

Tests: AgentServicesCompositionTests methods listed below.

Acceptance: one small coherent commit; no opportunistic adjacent migration.

Expected Tests

Test Name	Class	What It Verifies
AgentServicesComposition_RootResumedAndSubagent_ReuseExactBroker	AgentServicesCompositionTests	Verifies the named contract and leaves adjacent slices independently testable.
AccessBrokerProvider_RepositoryUsageMcpAndDevTunnel_ReturnExactHostBroker	AgentServicesCompositionTests	Verifies the named contract and leaves adjacent slices independently testable.
MainWindowViewModel_VsCodeTool_UsesHostBroker	AgentServicesCompositionTests	Verifies the named contract and leaves adjacent slices independently testable.