Clear ad-hoc keychain caches

[magnaprog]

Summary

• Clear both CodexBar-owned keychain services during ad-hoc rebuilds: com.steipete.CodexBar and com.steipete.codexbar.cache
• Silence security delete-generic-password output in the rebuild log
• Update the development setup doc to explain what this change covers and what still requires stable signing

Why

./Scripts/compile_and_run.sh was only clearing the legacy keychain service.
The current runtime cache under com.steipete.codexbar.cache was left behind, so a fresh ad-hoc-signed build could still run into stale ACLs and prompt again.

This change is limited to CodexBar-owned keychain entries.
Third-party items such as Claude Code-credentials still need stable signing if you want macOS to keep the Always Allow grant across rebuilds.

Validation

• ./Scripts/lint.sh lint
• bash -n Scripts/compile_and_run.sh
• ./Scripts/compile_and_run.sh --test reaches the updated keychain-clearing path, then stops because this environment has Swift 6.0.0 while the package requires Swift tools 6.2

Platform check

The app target in this repo is macOS-only, and the keychain cache path is guarded behind #if os(macOS).
I did not find the same issue in the non-macOS code paths in this package.