[3.0] Delete the deprecated IntelliTect.CredentialManager module folder and tests

## Summary

`IntelliTect.CredentialManager` was hard-deprecated in `IntelliTect.PSToolbox` **2.1.0** (removed from the umbrella's `RequiredModules`; manifest tagged `Deprecated`; `Write-Warning` on import; full migration guide in [`Modules/IntelliTect.CredentialManager/DEPRECATED.md`](../blob/main/Modules/IntelliTect.CredentialManager/DEPRECATED.md)). The PSGallery package `IntelliTect.CredentialManager` 1.1.1.1 will be unlisted when #117 is resolved.

In `IntelliTect.PSToolbox` **3.0**, the on-disk module should be deleted entirely. Tracking that here so we don't forget.

## Deliverables

- [ ] Delete `Modules/IntelliTect.CredentialManager/` (entire folder, including `.psm1`, `.psd1`, `DEPRECATED.md`)
- [ ] Delete `Modules.Tests/CredentialManager.Tests.ps1` (all tests are already commented out)
- [ ] Remove any stragglers from `PROJECT_OVERVIEW.md` §2.4 and the §6 deprecation matrix
- [ ] Remove any remaining mentions of the module from `README.md` and `CONTRIBUTING.md`
- [ ] Verify the [research report](https://github.com/IntelliTect/PSToolbox/blob/main/Modules/IntelliTect.CredentialManager/DEPRECATED.md) link in `DEPRECATED.md` either dies cleanly with the file or is preserved elsewhere
- [ ] Bump umbrella `IntelliTect.PSToolbox` to `3.0.0` in the release containing this change
- [ ] Release notes call out the deletion as a breaking change

## Why a major bump

Anyone still doing `Install-Module IntelliTect.PSToolbox` and relying on `Get-CredentialManagerCredential` / `Set-CredentialManagerCredential` etc. via the umbrella will already get `CommandNotFoundException` after 2.1.0 (module removed from `RequiredModules`). The 3.0 deletion completes the lifecycle — direct `Install-Module IntelliTect.CredentialManager` will still work against PSGallery 1.1.1.1 until #117-driven unlist lands.

## Depends on

- #117 (to actually unlist the PSGallery package)
- A `3.0` release milestone existing

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>