Support removing unused schema objects based on introspection

[SensibleWood]

Based on:

• This Slack: https://open-api.slack.com/archives/C023Y5YJ474/p1747759018643349
• Comments already made here: Change name of component schema item, delete schema item properties, create subset based on a schema #108

Pulling this into a separate discussion as a candidate for v2.0

Add support for removing unused objects through introspection as doing this retrospectively through Spectral/Redoc/Vacuum linting results and then another Overlay seems like a chore.

Proposed syntax:

- target: $.paths['/accounts/{AccountId}']
  remove: true
  remove-unused-references: true # default is false

Support for this seems like a good extension to Overlay because:

• Reduces the need for handcraft rules to do it.
• Reduced cognitive load on maintainers to keep up with changes to the OpenAPI description that may be transparent to them.
• Increases the utility of Overlay.

I think the access path for this is reasonably deterministic, however I would tempted to restrict to local references only - a recursive/walk-based into remote references would a) be difficult to prescribe and b) possibly be very difficult to implement.