refactor: rename SPIFFE_DEMO env var prefix to DOCSCLAW

[pavelanni]

Summary

The Viper env var prefix is still SPIFFE_DEMO from the original
Zero Trust demo. All env-based configuration requires the
SPIFFE_DEMO_ prefix (e.g. SPIFFE_DEMO_OTEL_ENABLED), which
is confusing for DocsClaw users.

Location

internal/config/config.go:88:

v.SetEnvPrefix("SPIFFE_DEMO")

Proposed change

Rename to DOCSCLAW:

v.SetEnvPrefix("DOCSCLAW")

This changes all env var mappings:

• SPIFFE_DEMO_OTEL_ENABLED → DOCSCLAW_OTEL_ENABLED
• SPIFFE_DEMO_SERVICE_PORT → DOCSCLAW_SERVICE_PORT
• etc.

Considerations

• Check if any deployment manifests, Helm charts, or docs
  reference SPIFFE_DEMO_* env vars and update them
• Consider accepting both prefixes during a transition period
  (check DOCSCLAW_* first, fall back to SPIFFE_DEMO_*)
• Also rename the config file paths (/etc/spiffe-demo/) at
  the same time