Support named argument syntax for model method run (e.g. --arg key=value)

Problem Statement

Running model methods currently requires passing all arguments as a JSON blob via --input:

swamp model method run workflow-runs pr --input '{"pr": 104, "json": false}'

For methods with simple scalar arguments, this is cumbersome — especially for frequent interactive use. Users need to remember the JSON structure, quote it correctly for their shell, and type boilerplate for every invocation.

Proposed Solution

Add a --arg key=value flag (repeatable) as an alternative to --input:

swamp model method run workflow-runs pr --arg pr=104
swamp model method run workflow-runs pr --arg pr=104 --arg json=true

The CLI would:

• Parse each --arg as key=value, coercing the value to the type declared in the method's Zod argument schema (number, boolean, string)
• Merge --arg values into an object, equivalent to --input
• Error if both --input and --arg are provided (or merge them with --arg taking precedence)
• Support dot notation for nested objects, consistent with --global-arg (e.g. --arg config.timeout=30)

Affected Components

• CLI argument parsing for swamp model method run
• Input resolution logic that currently handles --input and --input-file
• Type coercion layer (new) to convert string values to schema-declared types

Alternatives Considered

• Positional arguments: Too ambiguous for methods with multiple args
• --input 'pr=104': Overloading the existing flag could break JSON detection
• Shell aliases/wrappers: Pushes complexity to the user