chore: prepare release v3.7.9

[mekilis]

Summary

• Bump chart to 3.7.9 and default Convoy image tag to v26.6.6 (appVersion v26.3.7 -> v26.6.6)
• Applied across root chart, vendored agent/server subcharts, values defaults, Chart.lock, and README badges/table
• v26.6.6 carries the SSO admin portal auth fix plus the follow-up security hardening (SSO redirect validation, fail-closed slug auth, endpoint RBAC), so fresh installs off the chart default get the fully patched image

Test plan

• helm dependency update regenerates Chart.lock cleanly
• helm lint passes
• helm template renders agent/server deployments with getconvoy/convoy:v26.6.6
• Image getconvoy/convoy:v26.6.6 published on Docker Hub (amd64 + arm64)

[cursor]

Current version of PR was reviewed by /review-bugbot on Jul 20, 13:18 GMT+1. It flagged 0 findings.

^{Bugbot on commit f7891e8 is skipped.}