docs: fix groups add syntax in README (fixes #107)

[flupkede]

Fix

Corrects the codesearch groups add example in README.md to match the actual CLI behavior.

Issue

Fixes #107

What changed

• Line 356: codesearch groups add my-group repo1 repo2 repo3 → codesearch groups add my-group --aliases repo1 repo2 repo3

The CLI only accepts the group name as a positional argument. Members must be added via the --aliases (-a) flag.

Documentation audit

Full audit of all CLI examples in README.md against actual --help output — no other discrepancies found:

• ✅ All index add/rm/list/prune/symbol examples match CLI
• ✅ All serve, mcp, search, doctor, cache references match CLI
• ✅ All MCP configuration JSON snippets are correct
• ✅ CLI Reference table at line 378–391 is accurate
• ✅ CHANGELOG.md has no stale groups add syntax
• ✅ README_CSharp.md has no groups add references

QC

cargo fmt ✅ | cargo check ✅ | cargo clippy ✅ | 432/432 tests passed ✅