feat(contracts): add agent identity registry per ADR-0002

[moises-cisneros]

Closes #13

Summary

• New identity-registry Soroban contract implementing the ADR-0002 interface (sequential IDs from 0, unique URIs, owner metadata, dual-auth payment wallet).
• Typed IdentityError enum (codes frozen, 9–11 reserved), Registered/UriUpdated/MetadataSet events, TTL extend-on-write.
• Hardened .gitignore files (env, keys, secrets, Stellar identities).

Acceptance criteria

• Every public function from ADR-0002 exists, with the same name and signature
• Happy-path test for each public function
• Auth test without mock_all_auths()
• Duplicate registration returns the defined error, with test
• Events asserted in tests (topics and data)
• TTL extension test on the keys the ADR says
• stellar contract build, cargo test and cargo clippy --all-targets -- -D warnings pass

Verification evidence

cd contracts
stellar contract build          # pass, identity_registry.wasm 13268 bytes
cargo test -p identity-registry # pass, 15/15
cargo clippy --all-targets -- -D warnings  # pass
cargo fmt --all -- --check       # pass

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View logs

[XxHugheadxX]

Reviewed against ADR-0002 and #13. The contract is in good shape: all 23 public functions, the error codes (1–8, 9–11 reserved, 100), the three events with their topics, the storage keys and types, and extend_ttl match the ADR. The auth test runs without mock_all_auths(), and the TTL tests follow the ADR's method.

Gates, run locally (CI does not build contracts yet, #29)

stellar contract build                        ✅ identity_registry.wasm (12 KB)
cargo test                                    ✅ 15/15 (+ placeholder 1/1)
cargo clippy --all-targets -- -D warnings     ✅
cargo fmt --all -- --check                    ✅

Fixed in this branch (2 commits on top of yours, no force-push)

1. eb8a4a7 — TTL of the new URI index. set_agent_uri wrote UriIndex(new_uri) but did not extend its TTL, while ADR-0002 requires every write to extend the keys it writes. In tests the new index was left at 4,095 ledgers while AgentUri got 1,036,800, so the index could be archived long before the URI, breaking agent_id_by_uri and the duplicate check. The fix extends it, and set_agent_uri_updates_lookup_and_frees_old_uri now asserts the index TTL. The new assertion fails on the original code (left: 4095, right: 1036800) and passes with the fix.
2. 2654019 — removed openspec/. It is tooling configuration unrelated to #13 (scope rule in CONTRIBUTING). Feel free to propose it in its own PR.

All four gates pass again after both commits.

Optional, your call

• After unset_agent_wallet, get_metadata("agentWallet") returns Some(empty bytes) (asserted in test.rs). Stellar 8004 returns None there, since it reads the wallet slot. Returning None would keep clients written for Stellar 8004 working unchanged.
• MAX_METADATA_KEYS counts agentWallet, so 99 keys are left for the builder; Stellar 8004 does not count it. Fine either way, but worth a line in ADR-0002.
• The identity*.json pattern in contracts/.gitignore would silently ignore any deployment artifact named like that (#15). A narrower pattern may be safer.

@moises-cisneros please confirm the two commits look right to you; after that this is good to merge.