feat(skills): add an interchange skill chain

[TheGreatAxios]

Summary

• Add interchange, a hub skill that says what Interchange is, its mental model, and the rules for building on it.
• Add task-focused reference skills it routes to: interchange-agents, interchange-workflows (including a handoff table), interchange-run-modes (runLocal vs embedded vs remote hub, with a runnable runLocal example), interchange-client-apps (decision hub for apps that drive their own hub), interchange-embed-hub (hosting a hub in your process), interchange-hub-setup (app-side install procedure with a Transport and deploy snippet), interchange-hub-api (route and body table), and interchange-chat (mail as chat, run observation, approvals).
• All are background skills (no slash), loaded with skill_search and use_skill. Content comes from faremeter/interchange v0.4.0, Solution Builder, and workbench. Alpha caveats and unverified items are called out.
• Add the names to the telemetry allowlist and update PRODUCT, TELEMETRY, and the plugin manifest description.

Verification

• bun run check passes (7,870 tests).
• Client signatures were checked against packages/hub-client/src; the runLocal example against examples/workflow-quickstart; mail-as-trigger against hub-sessions/src/ws/sidecar-handler.ts.

Fixes CL-9677

[linear-code]

CL-9677