fix: untrack runtime config files and add them to .gitignore

These files are machine-specific and generated at runtime — they should
never have been committed. Remove from index (files kept on disk) and
add explicit gitignore rules to prevent future re-adds.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

config/ntp/chrony.conf

@@ -1,24 +0,0 @@
-# Personal Internet Cell - chrony Configuration
-# Provides NTP time synchronization
-
-# Allow NTP client access from local network
-allow 172.20.0.0/16
-allow 127.0.0.1
-
-# NTP servers to sync with
-server time.google.com iburst
-server time.cloudflare.com iburst
-server pool.ntp.org iburst
-
-# Local stratum for this server
-local stratum 10
-
-# Key file for authentication (optional)
-# keyfile /etc/chrony/chrony.keys
-
-# Drift file
-driftfile /var/lib/chrony/drift
-
-# Make chrony work as a server
-port 123
-bindaddress 0.0.0.0