feat: add canonical conversation session read-state ability

[chubes4]

Problem

Generic conversation-session clients can create, list, get, title, and delete principal-owned sessions through agents/* abilities, but cannot persist read state. The canonical session row exposes last_read_at, while WP_Agent_Conversation_Store explicitly excludes read-state mutation and no canonical ability owns it.

This leaves frontend adapters with two bad options: call a product-specific ability/store directly, violating layer purity, or return a successful mark-read response without persisting anything.

Evidence

• src/Transcripts/register-agents-conversation-session-abilities.php registers list/get/create/update-title/delete only.
• src/Transcripts/class-wp-agent-conversation-store.php documents last_read_at in the canonical row but says read-state is outside the transcript store contract.
• The built-in CPT store persists _agents_api_last_read_at but exposes no generic mutation path.
• fix: preserve workspace across chat session lifecycle frontend-agent-chat#123 needs workspace- and principal-scoped read-state behavior across the same lifecycle as the existing session abilities.

Required contract

• Add an optional generic read-state capability for stores that own it, rather than forcing it into every transcript store.
• Add a canonical ability such as agents/mark-conversation-session-read scoped by the existing workspace and resolved principal/session owner semantics.
• Verify ownership and workspace exactly as get/update-title/delete do.
• Return a truthful result including whether state was persisted and the canonical last_read_at; unsupported stores must fail explicitly rather than report success.
• Keep storage and UI policy out of the substrate.

Tests

• Create sessions with the same owner in two workspaces and prove marking one read cannot mutate or resolve the other workspace's session.
• Cover user and non-user principal owners through a principal-aware store.
• Cover unsupported stores with an explicit error.
• Confirm a subsequent get/list reflects the updated last_read_at and unread projection when the store supplies one.