Expand connector system and document local connector support

[lreading]

Summary

Post-v1 backlog item: expand the connector system beyond GitHub and define how local/custom connectors should work.

Potential scope:

• Add at least one new connector such as Atlassian or Bitbucket.
• Document how to create a connector clearly enough for contributors and agents.
• Consider a blueprint or abstract class.
• Support local connector usage without requiring an upstream PR.
• Validate connector shape before running it.

Alternatives considered

Keep only built-in GitHub support until the connector API is stable enough to document.

Affected packages

• app (presentation renderer)
• cli (@slide-spec/cli)
• docs (documentation site)
• shared (types and validation)
• Unsure