feat(auth,client): surface integration type and guide past root-page error

[4ier]

What

Two small nudges so first-time users don't have to parse a Notion paragraph to understand why root-page creation fails with an internal integration:

1. notion auth status and notion auth doctor now print the integration type (internal / public) derived from bot.owner.type on /v1/users/me.
2. When a validation_error matches the internal-integration root-page signature, client.errorHint appends a concrete workaround with the exact command to run next.

Why

Tracked in #25. The underlying API error is technically accurate but not actionable — this PR turns it into something copy-pasteable.

Before / After

notion auth status (internal integration)

Before:                        After:
✓ Authenticated                ✓ Authenticated
Workspace:      X              Workspace:      X
Bot:            cli            Bot:            cli
                               Integration:    internal
                               Root page:      not allowed (share a parent page)

Error wrap (internal integration hitting workspace root)

Error: validation_error: Provide a `parent.page_id` ...
  → Internal integrations can't create pages at the workspace root.
     Workaround: create (or pick) a parent page in the Notion UI, share
     it with this integration, then pass its ID as the parent:
         notion page create <shared-page-id> --title "..."
     To list pages shared with your integration: notion page list

Changes

• cmd/auth.go: detectIntegrationType(botInfo) reads bot.owner.type; unknown/missing shapes return "" so the UI gracefully omits the lines. Used in both auth status and auth doctor.
• internal/client/client.go: errorHint matches the Internal integrations aren't owned / insert_content signatures and returns a multi-line workaround.

Test plan

• TestDetectIntegrationType: workspace-owned → internal, user-owned → public, missing/unknown shapes → empty.
• Extended TestErrorHint cases for both signature variants.
• Updated setupAuthTest fixture to include bot.owner so auth status tests still pass and now exercise the new branches.
• Manual smoke against a real internal-integration account confirms both the auth status render and the live api POST /v1/pages error hint.

Closes #25