refactor: replace process-global runtime state with scoped dependencies

[abtreece]

Problem

Several important runtime dependencies are process-global mutable state rather than scoped dependencies.

Verified in:

• pkg/log/log.go:22-25, where tag and logger are package globals.
• pkg/log/log.go:112-179, where SetLevel and SetFormat mutate the global logger.
• pkg/template/client_cache.go:14-17, where the per-resource backend client cache is a package-global map plus mutex.
• pkg/template/client_cache.go:92-110, where shutdown closes all globally cached clients.

Why this is a problem

Global mutable state makes tests and runtime lifecycle harder to isolate. For logging, tests or components that mutate level/format affect the entire process. For backend clients, cache ownership is implicit: resource loading creates clients through a package-global cache, while shutdown later has to remember to call a package-global close function.

This is manageable today, but it becomes increasingly brittle as reload, per-resource backends, metrics wrapping, and tests grow.

Suggested refactor

Make runtime dependencies explicit:

• Introduce a scoped logger or logging configuration object passed into runtime components where practical.
• Introduce a ClientCache type owned by the top-level runtime/supervisor.
• Pass the cache into template resource construction instead of calling package-global cache functions.
• Keep package-level defaults only as thin compatibility shims if needed.

Acceptance criteria

• Tests can create isolated logger/cache instances.
• Per-resource backend clients are owned by a runtime-scoped cache.
• Shutdown closes the runtime-owned cache instead of a package-global cache.
• Existing behavior remains backward compatible for callers using default paths.