Add --log-level global flag with per-org info success messages

[Copilot]

Summary

Adds a global, optional --log-level persistent flag to the extension and wires it into the organization processing output. When the level is info, a success message is printed for each organization that is processed successfully.

This is a non-breaking, patch-level change. The flag is optional and defaults to warning, which preserves the existing output exactly.

Changes

• cmd/root.go: register the --log-level persistent flag (default warning, values info/warning/error) and parse it in PersistentPreRunE.
• internal/ui/log.go (new): log-level state (LogLevelInfo/LogLevelWarning/LogLevelError), ParseLogLevel, and a gated LogOrgSuccess helper that uses pterm.Success only when info logging is enabled. State is guarded by a sync.RWMutex so it is safe to read from concurrent workers.
• internal/processors/sequential.go & internal/processors/concurrent.go: on a successful organization result, call ui.LogOrgSuccess(org) so users running with --log-level info see Successfully processed organization '<name>' for each org.
• internal/ui/log_test.go (new): table-driven tests for ParseLogLevel and InfoEnabled.
• README.md: document the new flag under the "Other Flags" section.

Verification

• go vet ./... passes.
• go build ./... passes.
• go test ./... passes.
• gh security-config --help shows the new flag with the correct default.

Release Type

• Major - Breaking changes
• Minor - New features, backwards compatible
• Patch - Bug fixes, backwards compatible