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]>

src/agent/agent_loop.rs

@@ -21,6 +21,7 @@ use crate::context::JobContext;
 use crate::error::Error;
 use crate::extensions::ExtensionManager;
 use crate::history::Store;
+use crate::keys::KeyManager;
 use crate::llm::{ChatMessage, LlmProvider, Reasoning, ReasoningContext, RespondResult};
 use crate::safety::SafetyLayer;
 use crate::tools::ToolRegistry;
@@ -64,6 +65,7 @@ pub struct AgentDeps {
     pub tools: Arc<ToolRegistry>,
     pub workspace: Option<Arc<Workspace>>,
     pub extension_manager: Option<Arc<ExtensionManager>>,
+    pub key_manager: Option<Arc<KeyManager>>,
 }
 
 /// The main agent that coordinates all components.