feat: structured error output with error codes and context

Parent Epic: #63

Summary

Add error codes, context objects, and JSON error output so agents can programmatically handle failures.

Current State

All errors are plain text on stderr via console.error("Error: " + message). Agents must string-match to distinguish connection failures from validation errors from dispatch errors. Exit code is always 1.

Proposed Changes

Error class hierarchy (src/utils/errors.ts)

export class CliError extends Error {
  code: string;
  exitCode: number;
  context?: Record<string, unknown>;
  constructor(message: string, code = "E_CLI", context?: Record<string, unknown>) { ... }
}

export class ConnectionError extends CliError {
  constructor(message: string, context?: Record<string, unknown>) {
    super(message, "E_CONNECTION", context);
    this.exitCode = 3;
  }
}

export class MetadataError extends CliError { ... }      // E_METADATA, exit 4
export class ValidationError extends CliError { ... }    // E_VALIDATION, exit 2
export class AccountError extends CliError { ... }       // E_ACCOUNT, exit 5
export class TxDispatchError extends CliError { ... }    // E_TX_DISPATCH, exit 10

JSON error output (src/cli.ts)

When --output json is active, handleError emits structured JSON to stderr:

{"error":true,"code":"E_CONNECTION","message":"Failed to connect to wss://rpc.polkadot.io","context":{"chain":"polkadot"}}

Granular exit codes

Propagate error codes through codebase

Update existing throw new CliError(...) calls in tx.ts, query.ts, chain.ts, account.ts to use appropriate subclasses with context.

Files to Modify

• src/utils/errors.ts — extend error classes
• src/cli.ts — JSON error output in handleError, use err.exitCode
• src/commands/tx.ts — use TxDispatchError for dispatch failures
• src/commands/*.ts — use appropriate error subclasses

Verification

• Trigger each error type and verify echo $? returns the correct exit code
• Run with --output json and verify JSON error on stderr parses with jq

Start by reading src/utils/errors.ts and src/cli.ts, especially handleError, then trace the existing throw sites in src/commands/tx.ts and the other command files. Verify each listed failure category with the relevant command, check exit codes via echo $?, and confirm --output json produces stderr that parses with jq.