refactor(core,cli): migrate CLI parsers to relicario-core, parse.rs becomes shim

Plan B Phase 7 sub-step 2 — moves the bodies of parse_month_year,
base32_decode_lenient, guess_mime from crates/relicario-cli/src/parse.rs
to relicario-core. The CLI's parse.rs becomes a 19-line shim re-exporting
the new core API.

New core surface:
- time::MonthYear::parse (Result<_, RelicarioError>)
- mime::guess_for_extension (new mime module)
- item_types::TotpConfig::parse_secret — Zeroizing<Vec<u8>> wrapper
  over base32::decode_rfc4648_lenient

base32 module promoted from pub(crate) to pub so non-core consumers
(CLI shim, future Phase 8 WASM exports) can reach it. New
RelicarioError::InvalidMonthYear(String) for the parse error path
(mirrors sub-step 1's InvalidBase32). MonthYear::new keeps its
&'static str error type — bringing it to RelicarioError is DEV-A's P3.

CLI callsites unchanged (commands/{add,edit,attach}.rs); RelicarioError
auto-converts to anyhow::Error at ? boundaries.

cargo test --workspace: green (core 143, +7 from new tests)
cargo clippy --workspace: silent

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

crates/relicario-core/src/error.rs

@@ -129,6 +129,11 @@ pub enum RelicarioError {
     /// typed wrappers that delegate to it.
     #[error("invalid base32: {0}")]
     InvalidBase32(String),
+
+    /// Card-expiry month/year string failed to parse. Emitted by
+    /// [`crate::time::MonthYear::parse`].
+    #[error("invalid month/year: {0}")]
+    InvalidMonthYear(String),
 }
 
 /// Crate-wide result alias, reducing boilerplate in function signatures.