style(settings-nav): make model and agent primary tabs

[ewen-poch]

Problem

The Settings sidebar presented Agent as an indented child of Model, even though each opens an independent first-level panel. Model also used a generic sliders icon that did not communicate the panel's purpose.

Proposed change

• Keep Model first and place Agent second as peer Workspace tabs.
• Use Lucide Brain for Model and Bot for Agent.
• Remove the Agent-only tree guide, indentation, collapse state, and animation.
• Update renderer coverage and the macOS visual baseline for the intended navigation.

Scope and non-goals

• No settings content, navigation history, or panel behavior changes.
• No new icon dependency; both icons come from the existing Lucide package.
• No broader Settings redesign.

Acceptance criteria and validation

All listed checks ran after the final material source edit.

• Model and Agent render as ordered first-level tabs with the selected icons -> npm test -- src/renderer/src/pages/settings/SettingsPage.render.test.tsx -> passed, 52 tests.
• Final Settings navigation matches the intended visual hierarchy -> npm run build:e2e, then npm run test:e2e:visual -> passed, 1 visual flow.
• Renderer and main-process types remain valid -> npm run typecheck -> passed.
• Changed code satisfies lint gates -> npm run lint -> passed with 0 errors; 23 existing repository warnings remain.
• Repository unit and integration behavior remains green -> npm test -> passed, 649 test files and 9,583 tests; 15 files and 184 tests skipped.

The full test suite was rerun outside the restricted sandbox because its local HTTP fixtures require binding to 127.0.0.1. The initial sandbox run failed with EPERM for that environment restriction; the unrestricted rerun passed.

Review focus

• Model and Agent should share identical first-level row sizing, alignment, and active treatment.
• The removed collapse state should have no remaining callers.
• The updated visual baseline should contain Brain for Model and Bot for Agent.

Independent review was not run, per request. The macOS visual baseline covers the exact icon and hierarchy rendering; platform-specific rasterization outside macOS remains uncovered.

[github-actions]

Codex Review

Verdict: mergeable

No actionable findings.

Summary: Static inspection found no concrete merge-blocking defect in the pull request changes. Branch and pull-request title prechecks are valid.