Add workspace scope contracts for memory and transcripts

[chubes4]

Summary

• Adds AgentWorkspaceScope as the normalized workspace_type + workspace_id value object for generic workspace identity.
• Updates memory scope identity to include workspace fields and transcript store contracts to stamp sessions/dedup by workspace.
• Threads workspace scope through AgentConversationRequest so runtime transcript persisters can materialize workspace-stamped sessions.

Fixes #56.

Testing

• php tests/workspace-scope-smoke.php
• php tests/bootstrap-smoke.php
• php tests/conversation-loop-transcript-persister-smoke.php
• php tests/conversation-runner-contracts-smoke.php
• composer test
• git diff --check

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Source audit, contract implementation, tests, docs, and local verification. Chris reviewed and remains responsible for the change.