Add opt-in default CPT conversation store (closes #235)

[lezama]

Summary

Ships a WordPress-native default conversation store inside canonical, dormant behind a single filter, resolving the zero-store cold-start friction from #235.

Enable it with one line:

add_filter( 'agents_api_enable_default_conversation_store', '__return_true' );

When enabled, WP_Agent_Cpt_Conversation_Store registers the agents_api_session CPT and provides itself as a fallback on wp_agent_conversation_store at low priority (5) — any host store at the default priority still wins. When not enabled, nothing registers and a vanilla install is unchanged.

Why in canonical behind a flag (not a separate package)

This supersedes the earlier separate-companion direction, per maintainer consensus (@chubes4, @lezama) on the #235 thread:

• Easier to consume. One composer require wordpress/agents-api + one filter, instead of installing and version-tracking a second tiny plugin.
• No CPT shipped to every site. The store is dormant by default — a vanilla install has no post type and still returns the no-store error. An off-by-default dormant class is a far smaller WP-core concern than a mandatory session post type, so the core landing path (Convergence audit: merge parallel WordPress agents-api extractions #77) stays clean.
• No extra package to maintain for ~400 lines.

The store

WP_Agent_Cpt_Conversation_Store — wp_posts + wp_postmeta backed (no custom tables), generalized from the openclawp reference. Implements WP_Agent_Principal_Conversation_Store (and therefore WP_Agent_Conversation_Store) plus WP_Agent_Conversation_Lock. Ownership is keyed by (owner_type, owner_key) so audience/token principals are first-class; the int-user methods delegate to the _for_owner variants. Messages JSON in post_content, ownership/workspace/provider-continuity/context in meta, title in post_title. Single-writer lock via atomic add_post_meta($unique) + compare-and-swap reclaim.

Retention is intentionally not shipped — it stays a consumer responsibility so the substrate doesn't become a product runtime.

Changes

• src/Transcripts/class-wp-agent-cpt-conversation-store.php — the store.
• src/Transcripts/register-default-conversation-store.php — opt-in registration (CPT on init + filter fallback, both gated on agents_api_enable_default_conversation_store).
• agents-api.php — require both.
• register-agents-conversation-session-abilities.php — no-store error now points at the opt-in filter.
• docs/default-stores-companion.md — records the decision, marks the separate-package direction superseded.
• tests/cpt-conversation-store-smoke.php — 38 assertions via an in-memory WP shim.

Test plan

• php tests/cpt-conversation-store-smoke.php — 38 assertions: contract implementation, full CRUD round-trip, user/audience owner isolation, pending-session dedup, lock acquire/release/contention/CAS reclaim.
• composer test — full suite green (incl. bootstrap source-tree + no-product-prose checks).
• Reviewer: a wp-env pass on the meta_query/date_query list + dedup paths is worth doing before relying on it in production, since the smoke shims WP_Query.

Closes #235.