docs: record initial setup in changelog

Document the repository scaffold, architecture updates, and development tooling under Unreleased. Include the requirement to update the changelog for user-visible changes.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

AGENTS.md

@@ -67,6 +67,8 @@ The current project is a minimal executable and may not yet have tests.
 - Do not push changes unless explicitly asked.
 - Check the current branch before committing. Use conventional commit messages
   and include the repository's configured Copilot co-author trailer.
+- Add user-visible changes to the project's Unreleased changelog section using
+  its established format before you commit something, creating that section if needed.
 - Before every commit, including documentation-only commits, run
   `mise run pre-commit` and require a successful exit. Fix all reported
   formatting, lint, and test problems, then rerun the task until it passes.

CHANGELOG.md

@@ -0,0 +1,23 @@
+# Changelog
+
+All notable changes to this project will be documented in this file.
+
+## [Unreleased]
+
+### Added
+
+- Initial Rust workspace with the `runa` binary and a `runa-rune` library
+  scaffold for Phase 0. Rune integration is not implemented yet.
+- Project README and agent guidelines.
+- Base directories for architectural decisions, default image scripts,
+  development automation, examples, system tests, and the VS Code extension.
+- Mise tasks for formatting, Clippy linting, workspace tests, and sequential
+  pre-commit validation.
+- Tombi, rustfmt, and Clippy configurations.
+
+### Changed
+
+- Expanded the architecture specification with crate ownership and dependency
+  boundaries, and moved it to `docs/architecture.md`.
+- Required agents to pass `mise run pre-commit` before every commit and document
+  user-visible changes in this changelog.