Add Docker setup, .env.example, updated .gitignore/CHANGELOG and ADRs 004-007 + arc42 template

.github/instructions/AGENTS.md

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+# AGENTS.md
+
+Agents instructions for the linearvoid project.
+
+## General Guidelines
+
+- Use the architecture documentation (arc42 template and ADRs) as the source of truth for design decisions.
+- Follow the project folder structure defined in ADR-002 and reflected in the root README.
+- Update documentation first before implementing changes to ensure traceability of decisions.
+- Plan any task in the `TODO.md` file with clear steps and mark them as completed when done. Move completed sections to DONE.md. Write entry in root `CHANGELOG.md` for any significant changes. Avoid using in-memory todo lists or update them frequently to sync from the `TODO.md` file.
+- Before taking any action except for planning and writing `TODO.md`, always ask user for confirmation, accept `go`, `ok`, `yes`, `next`, or `continue` to proceed. If the user says `no`, `stop`, or `cancel`, abort the action and return to planning.