Add contributing workflow and expand gitignore

CONTRIBUTING.md

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+# Contributing
+
+## Branching Rule
+- Direct pushes to `master` are forbidden.
+- Every task must be implemented in a separate branch.
+- Finished work must be delivered through a merge request into `master`.
+
+## Standard Flow
+1. Update local `master`.
+2. Create a task branch from `master`.
+3. Implement the change in that branch.
+4. Run the relevant verification for the change.
+5. Commit the work in that branch.
+6. Push the branch to Gitea.
+7. Open a merge request into `master`.
+8. Merge only after review or explicit approval.
+
+## Branch Naming
+Use short, purpose-driven names, for example:
+- `feature/auth-password-reset`
+- `fix/session-revoke`
+- `docs/deployment-notes`
+- `chore/gitignore`
+
+## Minimum Expectations
+- Keep changes scoped to one task per branch.
+- Do not mix unrelated refactors into the same merge request.
+- Update docs when behavior, deployment, or operational flow changes.
+- If schema changes are introduced, include the Prisma migration in the same branch.
+
+## Agent Workflow
+- Codex or any other coding agent must create and use a dedicated branch per task.
+- After task completion, the agent should push that branch and prepare it for a merge request instead of pushing directly to `master`.