feat(web): show only authoritative subagent model metadata

[swear01]

Fixes #1221

Root cause

The existing subagent model badge only inspects model-bearing child blocks. That is sufficient for Claude sidechains, but Codex app-server collaboration events already preserve an authoritative explicit per-invocation model in the spawn tool input and may not expose child blocks in the parent trace.

Change

• Fall back to the explicit subagent invocation model only when no executed child model is available.
• Keep executed child-model data authoritative and first.
• Never fall back to the caller/session model, so Cursor/Grok/OpenCode cards remain blank when their ACP events do not expose per-subagent model metadata.

Impact

Codex and any other backend that explicitly reports the subagent invocation model now show it in the existing badge. No inferred model is displayed.

Validation

• bun typecheck
• bun run test — CLI 1527, hub 643, web 1562, shared 142 tests passed
• git diff --check
• bun run build:web
• isolated hub health check on port 43006

AI disclosure

Investigated and implemented with OpenAI Codex under human direction.

[github-actions]

Review mode: initial

No issues found in the diff.

Residual risk: I could not run bun test here because bun is not installed in this environment. The new helper coverage exercises the fallback path, but there is still no rendered-component test for the badge in ToolCard/ToolMessage.