feat: side session panel + new session model inheritance

[broodcicada-glitch]

What

Adds a side session panel for parallel chat alongside the active tab. Also fixes new sessions defaulting to a hardcoded model.

Side Session

• SideSessionPanel — draggable panel (280px-50vw) with full ChatInput + MessageList pipeline
• SessionChat — extracted MessageList + ChatInput shared by main and side
• vertical_split button in session header to toggle side panel
• uiStore.sideSessions — ephemeral per-tab memory map
• sessionStore — side flag, Sidebar filters side sessions
• ChatInput — sessionId prop + launchControls skip for side
• workspacePanelStore — 450px default for side-panel co-layout
• Compact fix — only workbench triggers compact, not side panel
• HERO fix — empty sessions now render ChatInput

Model Inheritance

• New sessions inherit model/provider from last active session
• Falls back to any existing selection on first-start

Files: 9 changed

[github-actions]

PR quality triage

Changed areas: area:desktop

CLI core policy: No CLI-core policy block detected.

Missing-test policy: Blocked by policy until a maintainer applies allow-missing-tests or matching tests are added.

Coverage baseline policy: No coverage-baseline policy block detected.

CLI core files:

• none

Coverage policy files:

• none

Required check plan:

• See PR Quality / scope-plan; it is the single source of truth for selected jobs.

Test coverage signals:

• BLOCKING unless allow-missing-tests is applied: Desktop product files changed without a desktop test file in the PR.

Risk notes:

• Desktop state/API layer changed: verify store persistence, WebSocket behavior, and startup errors.

Hard merge gates come from the deterministic GitHub Actions contract lanes above.