An agent session's workspace can no longer be changed after creation

What changed

When a user creates an agent session, the chosen workspace (directory) is locked in for the lifetime of that session. The session settings UI no longer offers a control to switch to a different directory.

The schema enforces this: UpdateSessionDto deliberately omits workspaceId; the DataApi rejects requests that try to PATCH it.

Why this matters to the user

Users who in v1 changed an agent's accessible path mid-conversation will not find that affordance in v2. The conversation history and session state stay tied to the directory they were created against.

What the user should do

To work on a different directory with the same agent, create a new session with the new workspace. Sessions sharing an agent share all cognitive config — only the workspace differs.

The session list groups sessions by parent agent, so multiple sessions per agent (one per workspace) remain manageable.

Notes for release manager

This is part of the workspace normalization theme — see also 2026-05-19-agent-session-primary-workspace.md (single workspace per migrated session) and 2026-05-20-agent-session-config-flattened-to-agent.md (per-session config moved to the agent).