docs: Add best-effort design doc to keepalive package (gt-gcnnr)

Enhanced the package-level documentation to explain why all write
operations in the keepalive package silently ignore errors.

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/keepalive/keepalive.go

@@ -1,4 +1,15 @@
 // Package keepalive provides agent activity signaling via file touch.
+//
+// This package uses a best-effort design: all write operations silently ignore
+// errors. This is intentional because:
+//
+//  1. Keepalive signals are non-critical - the system works without them
+//  2. Failures (disk full, permissions, etc.) should not interrupt gt commands
+//  3. The daemon tolerates missing signals by using timeouts
+//
+// Functions in this package write JSON files to .runtime/ or daemon/ directories.
+// These files are used by the daemon to detect agent activity and implement
+// features like exponential backoff during idle periods.
 package keepalive
 
 import (