[access-github-usage] - Migrate GitHub usage providers to broker leases

[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-github] - Implement approved environment and gh CLI token executor #1541.

Summary

Move the two GitHub usage providers from raw token delegates to one small typed lease adapter.

Current Limitation / Root Cause

GitHubCopilotUsageProvider and GitHubActionsUsageProvider still resolve raw GitHub token strings independently from repository and persistence consumers.

Affected Files

Path	Change
Phantom.Workspaces/Services/UsageProviders/GitHubCopilotUsageProvider.cs	Remove or update only the named bounded production surface.
Phantom.Workspaces/Services/UsageProviders/GitHubActionsUsageProvider.cs	Remove or update only the named bounded production surface.
Phantom.Workspaces/Services/UsageProviders/GitHubCredentialUsageAdapter.cs	Remove or update only the named bounded production surface.
Focused test file	Add only the tests listed below.

Design / Fix

internal sealed class GitHubCredentialUsageAdapter
{
    public ValueTask<TResult> UseAsync<TResult>(
        GitHubCredentialPurpose purpose,
        Func<string, CancellationToken, ValueTask<TResult>> operation,
        CancellationToken cancellationToken);
}

Each refresh submits its own Usage-purpose request, bounds plaintext to the HTTP operation, and disposes the result.

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-github-usage] - Migrate GitHub usage providers to broker leases

Production types: GitHubCopilotUsageProvider, GitHubActionsUsageProvider, GitHubCredentialUsageAdapter.

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

Expected Tests

Test Name	Class	What It Verifies
GitHubCopilotUsageProvider_Refresh_UsesBrokerLease	GitHubUsageProviderAccessTests	Verifies the bounded cutover with no remaining production reference.
GitHubActionsUsageProvider_Refresh_UsesBrokerLease	GitHubUsageProviderAccessTests	Verifies the bounded cutover with no remaining production reference.
GitHubUsageProviders_DifferentUses_HaveDistinctPurposeFingerprints	GitHubUsageProviderAccessTests	Verifies the bounded cutover with no remaining production reference.