RFC 0011: Add writable Windows and Linux native secret backends

[rmanibus]

Context

Issue #138 moved native secret write and existence checks into the shared internal/secretref backend abstraction.

The next step is to implement those writable capabilities for the non-macOS native backends so interactive CLI secret creation can use Windows Credential Manager and Linux Secret Service directly.

Parent issue: #93

Goal

Add Store/Exists support for wincred:// and secret-service:// backends through the shared writable backend contract.

Scope

• implement writable support for wincred://<target> using native Windows Credential Manager APIs
• implement writable support for secret-service://<collection>/<item> using the native Secret Service D-Bus API
• expose write support only where the backend is actually available in the current platform/runtime
• add backend tests and CLI integration coverage for the new writable paths

Acceptance Criteria

• wincred:// supports write and existence checks on Windows user-context runs
• secret-service:// supports write and existence checks when a Secret Service session/keyring is available
• unavailable backend/session cases remain typed and actionable
• interactive CLI secret prompting can offer native Windows/Linux storage through the shared backend abstraction
• no secret values are logged or printed

Out of Scope

• OAuth token/blob storage changes
• KWallet support
• non-native encrypted file fallback changes