feat: support default values for command tool arguments

[Minitour]

Summary

• Adds a default field to ArgumentDefinition so command tool arguments can specify fallback values.
• When a caller omits an argument that has a default, the executor uses the default for placeholder substitution instead of failing with a missing-argument error.
• Arguments with defaults are excluded from the MCP
  equired array and annotated with default in the JSON Schema, so agents see them as optional.
• The capa sh CLI merges defaults into resolved args before the required-check, keeping the shell experience consistent.

Test plan

• Define a command tool with default on one or more args in capabilities.yaml and verify:
  • Calling the tool without the argument succeeds using the default.
  • Calling the tool with the argument overrides the default.
• Run capa sh --help and confirm the default value is displayed.
• Verify that args with defaults are not listed as required in the MCP schema ( ools/list).
• Ensure existing tools without defaults are unaffected.

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