Setup/upgrade flag to activate runtime-backed WordPress chat (config, connector service, platform, secret) #779

Problem

The pieces from #772/#773 are merged but can't be switched on by setup or upgrade. Every activation step is currently manual:

1. runtime-chat adapter config: agent slug → runtime_id, read through WpCodingAgents_Inbound_Events::adapter_config( 'runtime-chat' ). No installer flag writes it.
2. The inbound-event-connector is installed, but README says the operator must start it "from the same runtime environment". No managed service or unit runs it, and it only reads Slack env today, plus the new WordPress delivery env.
3. WordPress delivery env (WP_CODING_AGENTS_INBOUND_WORDPRESS_ENABLED, _ENDPOINT, _SIGNING_SECRET_ENV, the secret itself, and WP_CODING_AGENTS_RUNTIME_ID) has to match on both the connector and the Roadie process. Nothing generates or shares the signing secret.
4. Roadie's platform list must include wordpress alongside the existing platform (roadie#297: --platform/ROADIE_PLATFORM accepts several). The rendered unit doesn't set it.
5. The wordpress:<context> channel must resolve in ROADIE_CHANNELS_CONFIG. Otherwise ensureRuntime() refuses: "Channel … is not configured".

Target state

One opt-in setup/upgrade flag (e.g. --wordpress-chat-agent <slug>) that:

• writes the runtime-chat config;
• mints and reuses a signing secret into a 0600 env file loaded by both processes;
• runs the connector as a managed service next to Roadie (systemd and launchd);
• adds wordpress to the rendered platform list and a channel entry for the bound context.

Without the flag, rendered output is byte-identical to today's.

Deliverable

A committed change with tests:

• render snapshots with and without the flag;
• the secret is shared and not regenerated on upgrade;
• the connector unit is present only when enabled.

CHANGELOG and VERSION are release-owned and stay as they are.