Add Serena configuration and project memories

(Set up Serena IDE integration with a Nix language server and create a
memory graph documenting package conventions, core structure, tech
stack, and task completion workflows.)

.serena/memories/task_completion.md

@@ -0,0 +1,11 @@
+# nix-overlay — task completion
+
+Done when ALL of these pass/hold:
+
+1. Build passes: `nix build .#<package>` (or `nix flake check` for cross-cutting changes). Never leave a package committed that doesn't build.
+2. Formatting: `nix fmt` clean (nixfmt; `flake.lock` excluded).
+3. New package is staged (`git add packages/<name>/`) before build — blueprint index requirement.
+4. README.md Available Packages table updated if package added/changed visibly.
+5. Commit: conventional commit format (use conventional-commit / caveman-commit skill). No work left uncommitted.
+6. Beads: work claimed via `bd update <id> --claim` before, `bd close <id>` after.
+7. Pushed: `git pull --rebase && bd dolt push && git push` then `git status` shows "up to date with origin". `bd dolt push` skip message is expected, not a failure. Do not stop before push succeeds.