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/cli/mod.rs

@@ -10,12 +10,14 @@
 //! - Checking system health (`status`)
 
 mod config;
+pub mod key;
 mod mcp;
 pub mod memory;
 pub mod status;
 mod tool;
 
 pub use config::{ConfigCommand, run_config_command};
+pub use key::{KeyCommand, run_key_command};
 pub use mcp::{McpCommand, run_mcp_command};
 pub use memory::{MemoryCommand, run_memory_command};
 pub use status::run_status_command;
@@ -78,6 +80,10 @@ pub enum Command {
     #[command(subcommand)]
     Tool(ToolCommand),
 
+    /// Manage NEAR blockchain keys
+    #[command(subcommand)]
+    Key(KeyCommand),
+
     /// Manage MCP servers (hosted tool providers)
     #[command(subcommand)]
     Mcp(McpCommand),