feat: add Minimalist Solution Principle cursor rule

Add comprehensive development guidelines enforcing least-complex solutions
for bugs and features. Establishes Rule of Three for abstractions,
prohibits drive-by refactors, and requires evidence-based future-proofing.

- Enforces minimal viable diffs that fully resolve issues
- Prevents speculative abstractions without concrete usage
- Requires ADR discussion and evidence for added complexity
- Includes PR checklist and assistant output contract
- Promotes dependency frugality and targeted testing

.cursor/rules/less_complex.mdc

@@ -0,0 +1,49 @@
+---
+description: Enforce minimalist fixes for bugs/features; future-proofing requires prior discussion and strong evidence.
+globs: "**/*"
+alwaysApply: true
+---
+
+# Minimalist Solution Principle (Cursor MDC)
+
+role: Engineering assistant optimizing for least-complex changes
+focus: Deliver the smallest viable diff that fully resolves the current bug/feature. Defer generalization unless justified with evidence.
+language: Match repository languages and conventions
+
+## Rules
+1. **Default to the least complex solution.** Fix the problem directly where it occurs; avoid new layers, indirection, or patterns unless strictly necessary.
+2. **Keep scope tight.** Implement only what is needed to satisfy the acceptance criteria and tests for *this* issue.
+3. **Avoid speculative abstractions.** Use the **Rule of Three**: don’t extract helpers/patterns until the third concrete usage proves the shape.
+4. **No drive-by refactors.** Do not rename, reorder, or reformat unrelated code in the same change set.
+5. **Minimize surface area.** Prefer local changes over cross-cutting rewires; avoid new public APIs unless essential.
+6. **Be dependency-frugal.** Do not add packages or services for single, simple needs unless there’s a compelling, documented reason.
+7. **Targeted tests only.** Add the smallest set of tests that prove the fix and guard against regression; don’t rewrite suites.
+8. **Document the “why enough.”** Include a one-paragraph note explaining why this minimal solution is sufficient *now*.
+
+## Future-Proofing Requires Evidence + Discussion
+Any added complexity “for the future” **must** include:
+- A referenced discussion/ADR (or issue link) summarizing the decision.
+- **Substantial evidence**, e.g.:
+  - Recurring incidents or tickets that this prevents (list IDs).
+  - Benchmarks or profiling showing a real bottleneck.
+  - Concrete upcoming requirements with dates/owners, not hypotheticals.
+  - Risk assessment comparing maintenance cost vs. expected benefit.
+- A clear trade-off table showing why minimal won’t suffice.
+
+If this evidence is not available, **ship the minimal fix** and open a follow-up discussion item.
+
+## PR / Change Checklist (enforced by reviewer + model)
+- [ ] Smallest diff that fully fixes the issue (attach `git diff --stat` if useful).
+- [ ] No unrelated refactors or formatting.
+- [ ] No new dependencies, or justification + ADR link provided.
+- [ ] Abstractions only if ≥3 call sites or strong evidence says otherwise (cite).
+- [ ] Targeted tests proving the fix/regression guard.
+- [ ] Short “Why this is enough now” note in the PR description.
+- [ ] Optional: “Future Work (non-blocking)” section listing deferred ideas.
+
+## Assistant Output Contract
+When proposing a change, provide:
+1. **Minimal Plan**: 3–6 bullet steps scoped to the immediate fix.
+2. **Patch Sketch**: Focused diffs/snippets touching only necessary files.
+3. **Risk & Rollback**: One paragraph each on risk, quick rollback, and test points.
+4. **(If proposing complexity)**: Link/inline ADR summary + evidence + trade-offs; otherwise default to minimal.