feat(statusline): surface fallback-model indicator

[KokaKiwi]

Checklist

• I have read the Contributor Guide
• I have read and agree to the Code of Conduct
• I have added a description of my changes and why I'd like them included in the section below

Description of Changes

Surfaces when the session's most recent request landed on an on-error fallback route, right in the Claude Code statusline, so developers can see fallback state without digging through logs.

edgee statusline's summary fetch was broken: it called /v1/sessions/{session_id}/summary, but the registered route also needs an org slug, and nothing ever set one. The renderer's "never crash" fallback hid this as a silent 404-to-bare-marker. EDGEE_ORG_SLUG is now set alongside EDGEE_SESSION_ID at every agent-spawn site (relay::spawn_agent/run_agent, plus all 8 direct-spawn launch/*.rs targets), and render.rs builds the correct 3-segment URL from it.

SessionSummary gains total_fallback_requests, last_request_is_fallback, and last_request_model. format_line appends ⚠ fallback: {model} (N this session) when the last request used one. Needs edgee-ai/api#1169 to land to populate those fields.

Related Issues

EDGEE-1597