fix(agents): resolve managed agent runtime avatars

[ashtonships]

Summary

• resolve linked managed-agent card stock avatars from the effective runtime catalog at render time
• preserve custom profile and persona avatars, including the existing fallback when catalog metadata is unavailable
• leave action/share avatars and unlinked persona cards unchanged

This is intentionally a display-only fix. It does not migrate or rewrite persisted profile identity or avatar data.

Related issue

N/A — no matching open issue or pull request found.

Testing

• node --import ./test-loader.mjs --experimental-strip-types --test src/features/agents/lib/agentCardAvatar.test.mjs (17 passed)
• pnpm typecheck
• pnpm exec biome check src/features/agents/lib/agentCardAvatar.ts src/features/agents/lib/agentCardAvatar.test.mjs src/features/agents/ui/UnifiedAgentsSection.tsx
• pnpm build:e2e
• macOS application bundle build

Screenshots are not included because this changes data-dependent avatar selection without changing layout or styling; the runtime transitions and custom-avatar preservation cases are covered by focused regression tests.

Deferred

Persisted avatar provenance and profile ownership are broader lifecycle concerns and are intentionally not changed in this PR.