feat: Add NEAR key management with transaction signing and policy engine

Implements hybrid-custody NEAR key management where the agent holds scoped
function-call keys for routine operations while high-value operations require
explicit user approval through the existing channel approval flow.

Core infrastructure:
- Ed25519 key generation/import via ed25519-dalek (not near-crypto)
- AES-256-GCM encrypted storage via existing SecretsStore
- Hand-rolled borsh-serializable NEAR transaction types
- NEP-413 intent signing and MPC chain signature support
- Configurable policy engine with transaction analysis pipeline
- Daily spend tracking with automatic midnight UTC reset
- Encrypted backup/restore with Argon2id KDF
- CLI subcommands: generate, import, list, info, remove, export, policy, backup, restore
- NEAR ed25519 secret key leak detection (Critical/Block)
- WASM sign-payload host function (keys never enter WASM memory)
- KeyManager wired into AgentDeps for agent-wide access

Security invariants: private keys never reach the LLM or WASM boundary,
signing happens in host Rust code with Zeroize on drop, every transaction
is analyzed before signing, most-restrictive policy rule wins.

Co-Authored-By: Claude Opus 4.6 <[email protected]>

Cargo.toml

@@ -90,6 +90,13 @@ sha2 = "0.10"
 blake3 = "1"
 rand = "0.8"
 
+# NEAR key management (ed25519 signing, borsh serialization, base58 encoding)
+ed25519-dalek = { version = "2", features = ["rand_core", "zeroize"] }
+borsh = { version = "1", features = ["derive"] }
+bs58 = "0.5"
+argon2 = "0.5"
+zeroize = { version = "1", features = ["derive"] }
+
 # Docker sandbox
 bollard = "0.18"