feat(config): add code-explorer default agent

[rmacomber]

Summary

- Add a default `code-explorer` agent for whiteboard/voice code walkthrough callbacks
- Keep the agent read-only with file/search/LSP tools only
- Document its output style for concise code explanations with file references

## Why

The whiteboard/ElevenLabs flow can request a `code-explorer` agent. Without a default definition, that callback fails because the agent does not exist in the shipped defaults.

## Testing

- `go test ./internal/config ./internal/daemon -run 'TestBootstrap|TestLoadCustomAgents|TestParseAgentFile' -count=1`
- Parsed `internal/config/defaults/agents/code-explorer.md` frontmatter locally and verified read-only tools
- Scanned the PR candidate for secret-like tokens and personal paths

[kirby88]

This seems like an superset of PR #37? can you clean it please and I'll merge after. Thank you

[rmacomber]

Cleaned. I rebased this branch onto current main after #37 merged, so PR #38 now contains only:

- `internal/config/defaults/agents/code-explorer.md`

Verification rerun:

```text
go test ./internal/config ./internal/daemon -run 'TestBootstrap|TestLoadCustomAgents|TestParseAgentFile' -count=1
ok  github.com/get-vix/vix/internal/config  0.048s
ok  github.com/get-vix/vix/internal/daemon  0.036s
```

I also rechecked the agent frontmatter/read-only tool list and scanned the PR file for secret-like tokens/personal paths; no findings.