docs: update Loki queries from host to hostname label

Update all LogQL examples, agent instructions, and scripts to use
the hostname label instead of host, matching the Prometheus label
naming convention. Also update pipe-to-loki and bootstrap scripts
to push hostname instead of host.

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

.claude/agents/investigate-alarm.md

@@ -41,13 +41,13 @@ Search for relevant log entries using `query_logs`. Focus on service-specific lo
 **Query strategies (start narrow, expand if needed):**
 - Start with `limit: 20-30`, increase only if needed
 - Use tight time windows: `start: "15m"` or `start: "30m"` initially
-- Filter to specific services: `{host="<hostname>", systemd_unit="<service>.service"}`
-- Search for errors: `{host="<hostname>"} |= "error"` or `|= "failed"`
+- Filter to specific services: `{hostname="<hostname>", systemd_unit="<service>.service"}`
+- Search for errors: `{hostname="<hostname>"} |= "error"` or `|= "failed"`
 
 **Common patterns:**
-- Service logs: `{host="<hostname>", systemd_unit="<service>.service"}`
-- All errors on host: `{host="<hostname>"} |= "error"`
-- Journal for a unit: `{host="<hostname>", systemd_unit="nginx.service"} |= "failed"`
+- Service logs: `{hostname="<hostname>", systemd_unit="<service>.service"}`
+- All errors on host: `{hostname="<hostname>"} |= "error"`
+- Journal for a unit: `{hostname="<hostname>", systemd_unit="nginx.service"} |= "failed"`
 
 **Avoid:**
 - Using `start: "1h"` with no filters on busy hosts