Add Linked Project Relationships

[Naomi-Gift]

Projects may belong to the same ecosystem, protocol, organization, or product suite, but the contract cannot express relationships.

Acceptance criteria:

• Project owner can link related project IDs.
• Linked projects must exist.
• Duplicate links are prevented.
• Links can be removed by owner.
• Tests cover linking, unlinking, nonexistent projects, and unauthorized callers.

[mubking]

please i would like to work on this issue

[grantfox-oss]

🦊 GrantFox — @mubking has been assigned to this issue as part of the Official Campaign campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #152)
2. Your PR will be reviewed by the HubDApp maintainers

Good luck! Track your progress on GrantFox.