feat: implement Phase 1 Axon protocol core package

Adds the foundational Go package implementing the full Axon protocol
signing and crypto spec per PROTOCOL.md:

- Event/Tag structs and all kind constants (KindProfile through KindJobFeedback)
- Byte-exact canonical_payload construction per the PROTOCOL.md layout table
- Tag sorting and canonical_tags SHA256 hash (duplicate detection included)
- Ed25519 sign/verify, challenge sign/verify
- X25519 key conversion from Ed25519 keypair (RFC 8032 §5.1.5 clamping +
  birational Edwards→Montgomery map for pubkeys)
- ChaCha20-Poly1305 encrypt/decrypt for DMs (nonce prepended)
- MessagePack encode/decode for events and wire messages

Test vectors written first in testdata/vectors.json covering canonical_tags,
canonical_payload, event_id, and signature verification — all deterministic
known-input → known-output pairs for cross-language validation in Phase 4.

13 tests, all passing.

1 files changed, 10 insertions, 0 deletions

diff --git a/go.mod b/go.mod
new file mode 100644
index 0000000..4d76e2e
--- /dev/null
+++ b/go.mod
@@ -0,0 +1,10 @@
+module axon
+
+go 1.25.5
+
+require (
+        github.com/vmihailenco/msgpack/v5 v5.4.1 // indirect
+        github.com/vmihailenco/tagparser/v2 v2.0.0 // indirect
+        golang.org/x/crypto v0.48.0 // indirect
+        golang.org/x/sys v0.41.0 // indirect
+)