refactor(secret): remove in-process secret resolver, document piping

[tuannvm]

Delete internal/secret/ (Vault, 1Password, command providers) and have config read directly from the process environment. Users inject secrets via external tools (op run, vault agent, K8s Secret + envFrom) — smaller attack surface, no app-side backend clients.

Add docs/secrets.md covering 1Password, Vault, K8s patterns plus security nuances (shell history, ps leakage, leak testing).

Summary

Changes

Testing

• Tests pass locally
• New tests added (if applicable)

Related Issues

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: ab91b480-2e79-45eb-bd01-dabc00faef2c

📥 Commits

Reviewing files that changed from the base of the PR and between adf7984 and 3e39488.

📒 Files selected for processing (14)

• .gitignore
• README.md
• docs/secrets.md
• internal/config/config.go
• internal/config/config_test.go
• internal/secret/command.go
• internal/secret/command_test.go
• internal/secret/convert.go
• internal/secret/onepassword.go
• internal/secret/onepassword_test.go
• internal/secret/resolver.go
• internal/secret/resolver_test.go
• internal/secret/vault.go
• internal/secret/vault_test.go

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch worktree-piping-secret

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