feat: add #[codama(skip)] directive for all enum variants

[amilz]

Summary

• Adds #[codama(skip)] support for account and error and instruction variants.
• Discriminator counting is preserved — skipped variants still occupy their slot.

Usage

#[derive(CodamaInstructions)]
enum MyProgramInstructions {
    Initialize { amount: u64 },
    #[codama(skip)]
    EmitEvent {} = 228,  // Internal CPI-only, excluded from IDL
    Update { amount: u64 },
    // Update gets discriminator 2 (not 1)
}

In practice (escrow program — skipping an internal CPI event emitter):

#[codama(skip)]
#[codama(account(name = "event_authority", signer))]
EmitEvent {} = 228,

Works on CodamaAccounts and CodamaErrors enums with the same semantics.

Test plan

• cargo fmt --check && cargo clippy --all-targets -- -D warnings && cargo test

[lorisleiva]

Thanks! Just a few comments to keep things consistent.

[amilz]

Good catches. I think bd39202 should cover those @lorisleiva.

[lorisleiva]

Thanks!