[access-stored-secret] - Implement platform-store and environment secret executor

[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-broker-core] - Implement typed dispatch dependency cycles and shutdown #1538.

Summary

Add one typed stored-secret request/result and an executor limited to platform-store and environment sources.

Current Limitation / Root Cause

SecretProvider currently reads selected sources after a batch dialog and can silently retain old source semantics.

Affected Files

Path	Change
Phantom.Workspaces.Llm.Interfaces/Access/StoredSecretAccessContracts.cs	Implement the bounded production change owned by this issue.
Phantom.Workspaces/Services/Access/StoredSecretAccessExecutor.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 StoredSecretAccessRequest : CredentialAccessRequest<StoredSecretAccessResult>
{
    public required SecretReference Reference { get; init; }
}
internal sealed class StoredSecretAccessExecutor : ICredentialAccessExecutor<StoredSecretAccessRequest, StoredSecretAccessResult>;

Inspect reports only source type/existence. Execute reads the exact selected source after release commit and returns RenewalNeeded if it disappeared; no source switching.

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-stored-secret] - Implement platform-store and environment secret executor

Production scope: Phantom.Workspaces.Llm.Interfaces/Access/StoredSecretAccessContracts.cs, Phantom.Workspaces/Services/Access/StoredSecretAccessExecutor.cs.

Tests: StoredSecretAccessExecutorTests methods listed below.

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

Expected Tests

Test Name	Class	What It Verifies
StoredSecretExecutor_InspectAsync_DoesNotReadPlatformOrEnvironmentValue	StoredSecretAccessExecutorTests	Verifies the named contract and leaves adjacent slices independently testable.
StoredSecretExecutor_ExecuteAsync_ReadsExactSourceAfterReleaseCommit	StoredSecretAccessExecutorTests	Verifies the named contract and leaves adjacent slices independently testable.
StoredSecretExecutor_SelectedSourceMissing_ReturnsRenewalNeeded	StoredSecretAccessExecutorTests	Verifies the named contract and leaves adjacent slices independently testable.