feat: template provider

[dirathea]

Summary

Implement new template provider to let users compose secrets from other secrets

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Note

Introduces cross-provider templating and refactors provider plumbing.

• New template provider: Compose secrets using Go templates with uses-scoped dependencies; added docs and examples.
• Provider API refactor: Fetch now takes SecretContext; added resolver interfaces/types and per-provider secrets map in collector; updated redaction and all providers/tests accordingly; CLI registers template.
• Config/Docs: Add uses to provider config; update README/CONFIGURATION with template provider section.
• CI: Split workflows—ci.yml runs short tests; new end2end.yml runs full E2E against real services.
• Tests: Add comprehensive template provider E2E; update existing E2E to skip in short mode and adapt to new context API.

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