Codex-generated skill preambles hardcode ~/.Codex while installer exposes ~/.agents

## Summary

In a Codex session using gstack 0.16.2.0 at commit `dbd7aee5b6b5bb41b17ae7747568e2fda8a91d77`, the generated `review/SKILL.md` preamble invokes `~/.Codex/skills/gstack/bin/gstack-config`. The installed skill and binaries are under `~/.agents/skills/gstack`, so every such invocation fails.

## Exact error

```text
/bin/bash: /home/maxlab/.Codex/skills/gstack/bin/gstack-config: No such file or directory
```

## Reproduction

1. Install/expose gstack to Codex under `~/.agents/skills/gstack`.
2. Invoke the generated `review` skill.
3. Run the required preamble.
4. Observe calls to `~/.Codex/skills/gstack/bin/gstack-config` fail even though `~/.agents/skills/gstack/bin/gstack-config` exists and is executable.

Proof:

```text
ls: cannot access /home/maxlab/.Codex/skills/gstack/bin/gstack-config: No such file or directory
-rwxr-xr-x /home/maxlab/.agents/skills/gstack/bin/gstack-config
```

## Root cause

The Codex-generated SKILL body hardcodes a host path/capitalization that does not match the actual installed skill root supplied to the session. This is related to #1882 and #1163, but the concrete mismatch here is `.Codex` versus `.agents`, not only a renamed checkout.

## Impact

Skill config, telemetry, learnings, timelines, and preference prompts silently fail or emit shell errors. The review itself can continue only by manually substituting the actual install root.

## Expected

Generated skill bodies should resolve their own package root or use the actual host install root from setup, with a regression test that executes the preamble from a Codex `~/.agents/skills` install.

Start with the generated review/SKILL.md preamble and trace how the installer exposes ~/.agents/skills/gstack and its bin/gstack-config entry point. Reproduce the preamble from a Codex ~/.agents/skills installation, then add coverage proving the generated commands use the installed root and no longer reference ~/.Codex.

Written by the indexing model from the issue text.