feat(pgp): support delegated OpenPGP providers

[pando85]

Summary

• add an optional provider-agnostic OpenPGP API backend alongside APS' internal key manager
• delegate private-key decryption to the selected provider for password, autofill, and passkey flows
• retrieve and validate public certificates as needed while keeping encryption local with PGPainless
• preserve hierarchical .gpg-id recipient policy, including multi-recipient passkey creation
• handle OpenPGP permission/user-interaction continuations through the Activity Result API

Security and behavior

• APS never retrieves or imports provider secret-key material
• fetched certificates are rejected if they contain secret key material and are checked against the requested key ID before local import
• foreground provider interaction is coroutine-scoped so background passkey/index work cannot consume another activity's interaction
• discarded provider output and passkey ciphertext/plaintext error paths are zeroized
• provider bind/interaction failures fail closed instead of silently falling back to a local private key
• the internal PGP backend remains the default and can still be selected explicitly

This deliberately implements the OpenPGP API as a delegated cryptographic backend rather than a remote KeyManager: ACTION_GET_KEY is used for public certificates only, while private-key operations remain inside the provider. Provider-side encryption/signing is not required for pass-compatible APS operation, so encryption remains local using the resolved public certificates.

Validation

The branch has been exercised with:

• spotlessCheck
• :app:testDebugUnitTest
• :passkeys:core:test
• :passkeys:provider:testDebugUnitTest
• :crypto:pgpainless:test

The repository's normal PR workflow additionally runs the complete slim unit-test suite, APK assembly, lint, codestyle, and the passkey compatibility suite.

Closes #152

[forkline-bot]

The lint error (AvoidUsingNotNullOperator at OpenPgpProviderRepository.kt:151) was already fixed in commit 9963d00 — replaced importFailure!! with a safe ?.let pattern. No !! operators remain in the source. CI should pass on the current HEAD.

[forkline-bot]

✅ Fix pushed:

Fixed two lint issues:

1. Replaced runCatching with a specific try/catch(Exception) at OpenPgpApiBackend.kt:277 to satisfy the DenyListedApi lint rule
2. Removed 4 stale InvalidPackage entries from lint-baseline.xml that were no longer matching (LintBaselineFixed)

[forkline-bot]

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