fix: suppress embedded-repo warning with advice config (#14)

[olavostauros]

Summary

The modules add embedded-repo warning (#14) was caused by git add of a clone directory in the old hash-based subdirectory layout (pre-opacity redesign). git add on a specific file like .modules/manifest cannot trigger this warning — git only emits it when adding a directory that contains a .git subdirectory, or during git add ./wildcard operations. The current add task only stages .modules/manifest (no directory adds), and modules/ is gitignored by setup, so the warning cannot reproduce with current code.

This PR adds two layers of protection:

1. -c advice.addEmbeddedRepo=false on the git add call in the add task — defense-in-depth that suppresses the advice at the source, even if a future refactor inadvertently reintroduces a directory git add. The config key was introduced in git 2.35 (2022) and all supported platforms ship it; older gits silently ignore unknown -c keys.

2. Regression guard in test/add.bats — asserts no "embedded git repository" substring appears in modules add output.

Closes #14.

Validation

• mise run test — all 136 pass (including new guard)
• codebase lint "$PWD" — all 9 rules pass
• readme build --check — README is up to date
• git diff --check — no whitespace issues