fix: bind queued messages to session owners

[chubes4]

Summary

• authorize chat start, enqueue, and queue claims against the existing canonical conversation session store for the supplied workspace and principal
• serialize transient run and queue read-modify-write operations through an optional generic atomic store capability
• implement the default mutex with bounded, non-autoloaded rows in the existing WordPress options table using create-only insertion, token plus expiry, conditional stale takeover, transactional token fencing, and token-checked release
• coordinate explicit workspaces through the canonical main-network options table while preserving workspace state in the existing network option store
• reserve client run IDs before handlers execute, including requests that do not yet have a session, so duplicate same-owner requests cannot both run
• fail closed on ownership, lock, storage, and corrupt-state failures while preserving unowned site-local compatibility for existing stores
• keep the substrate generic with no product coupling, custom production tables, or schema migrations

Concurrency Coverage

• independent MariaDB connections serialize site and workspace state mutations
• expired lease holders cannot overwrite a conditional takeover winner
• foreign-owner and same-owner duplicate run_id races admit exactly one execution
• pre-session run reservations admit exactly one arbitrary handler and bind the winner to its canonical session
• double queue claims return every message exactly once without resurrection
• enqueue concurrent with claim preserves every message exactly once
• stale takeover, active contention timeout, token-checked release, and exception-safe release
• corrupt and structurally malformed state fail closed without overwrite
• single-site workspace fallback and WordPress option-cache coherence

Tests

• composer test
• composer phpstan
• composer validate --strict
• php tests/agents-chat-ability-smoke.php
• php tests/default-agents-chat-handler-smoke.php
• php tests/provider-turn-adapter-smoke.php
• php tests/chat-run-control-smoke.php
• php tests/chat-run-control-multisite-smoke.php
• php tests/bootstrap-version-skew-smoke.php
• AGENTS_API_WP_ROOT=/var/www/extrachill.com AGENTS_API_DB_HOST=... AGENTS_API_DB_NAME=... AGENTS_API_DB_USER=... AGENTS_API_DB_PASSWORD=... php tests/run-control-option-store-integration.php
• PHP syntax checks and git diff --check

Fixes #451

[chubes4]

Architecture constraint for this fix: no custom tables or migrations, and no parallel session-owner registry when the canonical conversation store can resolve ownership. Start/enqueue/claim should validate against the existing workspace-scoped principal-aware session owner; unsupported ownership resolution must fail closed. Agents API remains generic and Core-oriented.