crypto: switch back to in-process rust age crate once it gains ML-KEM

[fosskar]

Blocked on upstream: the Rust age crate lacks ML-KEM, so ssync-crypto shells out to the age CLI for PQ-hybrid keys (age-keygen -pq, ML-KEM-768 + X25519). The disabled in-process backend already exists behind the rust-age feature (crates/ssync-crypto/src/rust_age.rs) with the same AgeIdentity API, so switching back is a one-liner.

Drops the age CLI runtime dependency (and one subprocess spawn per encrypt/decrypt).

Refs: TODO.md Open #9, docs/DECISIONS.md §7, docs/identity.md.