WP AI Gateway as an opt-in model source for Pi (alongside the Roadie credential pool) #774

Problem

The optional WP AI Gateway integration only covers OpenCode, which is retired here. Pi, the active Roadie and Homeboy coding runtime, can't use it.

• lib/ai-gateway.sh: ai_gateway_enabled_for_opencode() returns true only when the runtime is opencode.
• Setup writes an OpenCode provider.wp-ai-gateway entry and nothing else.
• Today Roadie-managed Pi runs on Roadie's credential pool (ROADIE_PI_MODEL=roadie/opus in the rendered unit).

Hosts should get a choice of model source for Pi sessions, including sessions behind the WordPress chat platform (#771/#773):

1. Roadie credential pool (current default): subscription-backed and pooled in Roadie.
2. WP AI Gateway: the gateway's OpenAI-compatible /wp-json/wp-ai-gateway/v1 endpoint. Upstream keys stay in WordPress, and usage goes through the WordPress AI client. Roadie and Pi hold only a minted gateway token.

Target state

• --with-ai-gateway applies to Pi.
  • Setup and upgrade write a Pi provider/model entry pointing at the gateway base URL (respect Allow a full WP AI Gateway endpoint override #357's endpoint override if present), with the token from the existing mint/reuse flow.
  • The Roadie unit's Pi model is set to the gateway model.
• Without the flag, Pi stays on the Roadie credential pool, exactly as today.
• Keep the existing recursive-topology guard: a gateway route must not point back at the gateway.
• Remove the retired OpenCode-only gating, or generalize it, rather than adding a parallel path.

Check how Pi reads custom OpenAI-compatible providers (its models/providers config) before choosing the file to write.

Deliverable

A committed change with tests in tests/ai-gateway.sh:

• Pi with the gateway gets the provider entry and model env.
• Pi without the gateway keeps the roadie/* model and has no gateway entry.
• The token is reused across upgrades.
• The topology guard still holds.

Update the bridge snapshots if the rendered unit changes. CHANGELOG and VERSION are release-owned and stay as they are.