Support alternative CLI backends (Gemini CLI) for cross-model second opinions

## Problem

12+ gstack skills hardcode `codex exec` as the cross-model second opinion engine:

- `/codex`, `/review`, `/ship`, `/plan-eng-review`, `/plan-ceo-review`, `/plan-design-review`, `/design-consultation`, `/design-review`, `/office-hours`, `/autoplan`

These calls use Codex-specific flags (`-s read-only`, `-c 'model_reasoning_effort=...'`, `--enable web_search_cached`, `resume `) that have no direct equivalent in other CLIs.

Users who don't have (or don't pay for) OpenAI Codex CLI but do have Google Gemini CLI installed currently have to maintain a translation shim to use these skills. The shim works for basic calls but can't fully translate:

- `codex exec resume ` (session continuity — no Gemini equivalent)
- `-c 'model_reasoning_effort=...'` (dropped silently)
- `--enable web_search_cached` (dropped — Gemini has built-in search but no equivalent flag)
- `--json` JSONL output mode (Gemini doesn't support structured trace output the same way)

## Related

- #463 addresses fallback to Claude subagent when Codex is missing, but only for `/office-hours`
- Several skills already have `CODEX_NOT_AVAILABLE` detection but handle it inconsistently (some skip silently, some fall back to Agent tool)

## Proposal

Add a configurable "outside voice" backend in `~/.gstack/config.yaml`:

```yaml
outside_voice:
backend: gemini # or: codex, claude-subagent
# backend-specific config inherited from each CLI's own auth
```

Then in skill files, replace direct `codex exec` calls with an abstraction that dispatches to the configured backend. Each backend adapter handles flag translation:

| Codex flag | Gemini equivalent | Claude subagent |
|---|---|---|
| `-s read-only` | `--sandbox` | Agent tool (read-only by default) |
| `-c 'model_reasoning_effort=...'` | n/a (drop) | model param |
| `--enable web_search_cached` | built-in (drop) | WebSearch tool |
| `resume ` | n/a | SendMessage to existing agent |
| `--json` (JSONL traces) | n/a | parse agent output |

This would:
1. Let users choose their preferred second-opinion CLI
2. Unify the inconsistent `CODEX_NOT_AVAILABLE` handling across skills
3. Make the "outside voice" pattern a first-class gstack concept rather than a Codex-specific feature

## Workaround

A bash shim at `~/.local/bin/codex` that intercepts `codex exec` and translates to `gemini -p` works for most cases but breaks on session continuity and JSONL output parsing.

## Environment

- macOS (Apple Silicon)
- `@google/gemini-cli@0.35.1` via npm
- gstack 0.13.0.0