cmd/sgai: add embedded IDE workspace tab

[ucirello]

Summary

• add an embedded IDE tab that launches or reuses one workspace-scoped code-server session through the Sgai reverse proxy
• mint per-tab IDE access while keeping IDE traffic loopback-bound and scoped to the current workspace session model
• cover the IDE proxy, session lifecycle, route-entry reuse, and second-tab access paths with Go and React regression tests

Test Plan

• bun run build
• make build
• make lint
• bun test
• make test
• tmux + Playwright smoke at http://127.0.0.1:63816/workspaces/ide-check/ide with evidence under .sgai/retrospectives/2026-03-31-18-15.dwpa/screenshots/