Conversation sessions need a queryable product scope for project-bound transcripts

[chubes4]

Problem

WordPress Build can store product target metadata on chat sessions, but the generic conversation-session list path only filters by owner, agent, and context. A product cannot ask for "the latest chat session for project X" through the canonical session surface.

Evidence

• src/Transcripts/register-agents-conversation-session-abilities.php:201-204 accepts arbitrary metadata on create.
• src/Transcripts/register-agents-conversation-session-abilities.php:595-598 documents metadata as JSON-serializable product-specific data.
• src/Transcripts/class-wp-agent-cpt-conversation-store.php:236-256 lists sessions with owner/workspace plus optional agent_slug and context filters only.
• src/Transcripts/class-wp-agent-cpt-conversation-store.php:151 and src/Transcripts/class-wp-agent-cpt-conversation-store.php:225 persist metadata as opaque JSON, so nested product keys are not queryable by the generic list.

Needed Primitive

Add a runtime-neutral way to scope/query conversation sessions, for example:

• a canonical session_scope / thread_scope string on session create/list; or
• a constrained metadata filter/index contract for namespaced product keys.

Acceptance Criteria

• A caller can create a session scoped to an opaque product key like wp-build-project:123 without Agents API knowing what that means.
• The list/bootstrap path can return sessions for that scope only.
• The shape remains product-neutral and works for non-WordPress-Build callers.

Downstream WP Build issue will link here; the product need is project-scoped transcripts so switching projects switches/restores the right chat.