feat(#407): add full agent editor panel with sidebar sub-tree and all config fields

[sytone]

Closes #407 (Phase 1 + Phase 2)

What's in this PR

Sidebar sub-tree (Phase 1)

• MainLayout.razor: Agents nav item expands to list all registered agents when on any /agents/* route
• Live refresh via Hub.OnAgentsChanged — new agents appear immediately
• Clicking an agent navigates to /agents/{agentId}
• + Add Agent entry at the bottom navigates to /agents/new

Full agent editor (Phase 2)

• AgentDetailPanel.razor: new full-page component covering ALL AgentDefinitionConfig fields
• Agents.razor: adds @page "/agents/{AgentId}" and @page "/agents/new" routes
• Sections (collapsible via <details>): Identity & Model, System Prompt, Tools & Sub-Agents, Memory, Soul, Heartbeat, File Access, Access Controls, Runtime, Tool Policy
• Dirty tracking via JSON snapshot comparison — Save button disabled when clean
• Save via PUT /api/agents/{id}, auto-dismiss success after 4s
• Inline delete confirmation (no overlay)
• World default badges from GET /api/config/agents/{id}/effective
• Back to list button

Tests

• 4 new bUnit tests in AgentDetailPanelTests.cs
• 338/339 pass (1 pre-existing flaky mobile JS test AgentDropdown_NotRendered_WhenIsMobileIsTrue unrelated to this PR)

[sytone]

CI Health Monitor

Actions taken: Synced branch to main (was 1 commit behind).

Sync result: Clean merge — no conflicts. CI re-triggered automatically.

CI status after sync: All checks passing

• Analyze (csharp): pass
• impacted-tests: pass
• CodeQL, Code Pattern Checks, Dependency Security Audit, Secret Scanning: all pass

No action required on CI.