Resolve gopass:// password references

[1995parham]

What

Adds support for gopass-style cross-secret password references. An entry whose password is gopass://<path> now resolves, at read time, to the password of the entry at <path> (relative to the store root), so a canonical credential need not be duplicated. Rotating the canonical entry automatically updates every reference.

Behaviour

• Substitutes the referenced entry's password into the referencing entry.
• Falls back to the referenced entry's username / OTP only when the referencing entry does not define its own; any other fields on the referencing entry are preserved.
• Recursive (a target may itself be a reference), with cycle detection, a depth limit, and path-traversal guards.
• Works in both the decrypt screen and autofill. Editing an entry still shows the raw gopass:// reference rather than the resolved password.
• Referenced entries that require a different PGP key/passphrase are unlocked interactively, reusing the existing passphrase/biometric/PIN flow. Non-reference entries are completely unaffected.

Implementation

• PasswordEntry.isReference() / referencePath() — pure detection in format:common.
• PasswordReferenceResolver — UI/crypto-agnostic orchestration + a pure, unit-tested merge step. Decryption of the referenced file is injected by the caller.
• BasePGPActivity gains a reference-decrypt sink so the shared unlock state machine can decrypt an arbitrary referenced file and hand back its plaintext; decryptWithPassphrase and the passkey/creation activities are untouched.
• DecryptActivity and AutofillDecryptActivity resolve references before rendering/filling.

Tests

• PasswordEntryTest — reference detection (scheme case-insensitivity, first-line-only, non-references).
• PasswordReferenceResolverTest — password substitution, username/OTP fallback vs. origin's own fields, nested chains, cycle detection, depth limit, path-traversal rejection, aborted-unlock fallback.

All unit tests and spotlessCheck pass. Runtime flows (different-key prompt, circular reference, edit-shows-reference) should be verified on device.