docs(workflow): use GitHub Issues for internal work

[ahilles107]

What

Use GitHub Issues for all Gravity tasks, including internal maintainer work. Add repository instructions for coding agents, explicitly overriding inherited Linear requirements, and align contributor and release guidance with GitHub issue references.

Fixes #5. Follow-up to merged #4.

Why

The public contributor guide already directs users to GitHub, while shared agent guidance still requires Linear. Repository-local instructions make the workflow consistent without changing other projects' settings. Existing private backlog items require a separately scoped disclosure review before migration.

How tested

• All six required hosted checks passed, including visual CI.
• Reviewed tracked guidance and workflows for Linear dependencies; retained the Linear-specific secret detector.
• Documentation links and git diff checks passed. Gitleaks on the feature commit found no secrets.
• Full local verification passed through native Clippy. Native test compilation failed with No space left on device; local visual tests were not reached. Native tests passed in the preceding skills audit; this change only edits documentation.

Documentation only; no release or production configuration change.