fix(registry): make owner Signer to prevent update_field signer privilege escalation (#4)

[nanookclaw]

Problem

Closes #4.

register_agent fails with Cross-program invocation with unauthorized signer or writable account when both conditions are true:

1. owner != payer (minting on behalf of a different wallet)
2. additional_metadata is provided (non-null, non-empty)

Root cause

Step 2g calls spl_token_metadata_interface::instruction::update_field with owner as the update_authority. That instruction marks the update_authority as a required signer:

AccountMeta::new_readonly(*update_authority, true)  // is_signer = true

But owner was declared as UncheckedAccount<'info>, which means its AccountInfo has is_signer = false. The Solana runtime detects the mismatch and rejects with:

signer privilege escalated
Program ... failed: Cross-program invocation with unauthorized signer or writable account

The default path (owner == payer) was unaffected because payer: Signer<'info> already had is_signer = true, which propagated to both account slots.

Fix

Change owner from UncheckedAccount<'info> to Signer<'info>.

- /// CHECK: Can be any valid pubkey
- pub owner: UncheckedAccount<'info>,
+ /// Must sign: update_field CPI requires owner as update_authority signer.
+ /// When owner == payer, pass payer pubkey for both accounts.
+ pub owner: Signer<'info>,

This is semantically correct: owner is always the metadata update authority (set in step 2f's initialize_metadata call). Requiring the owner to sign authorises registration of their own agent NFT.

Client behaviour

Case	Before	After
owner == payer	✅ Pass payer for both; one signer	✅ Same — no change
owner != payer, no metadata	✅ No update_field CPIs; no issue	✅ Owner must now sign (correct)
owner != payer, with metadata	❌ Signer escalation crash	✅ Owner signs; CPIs succeed

Tests

Two regression tests added to programs/sati/tests/registry/register_agent.rs:

• test_register_agent_owner_not_payer_metadata_key_too_long_correct_error — owner != payer, owner included as signer; transaction reaches input validation (MetadataKeyTooLong) rather than the signer escalation bug. Asserts the error does not contain "signer privilege".
• test_register_agent_owner_equals_payer_metadata_key_too_long_correct_error — owner == payer; verifies the common path still works after the Signer change.

The full success-path integration test requires a Token-2022 devnet environment with update_field dispatch; the TypeScript SDK tests cover that path.