Add structure guidance rules

Status summary#

Complete in draft PR #2305. Four structure rules now cover conditional shape, small helpers, truthiness, and validation of unknown values. The first 20-file sample was reviewed and its counterexamples were folded back into the conditional and unknown-shape rules.

Spec#

• Add the submitted structure guidance. The helper and truthiness rules retain their submitted wording; review feedback refined the conditional rule and added unknown-shape validation.
• Remove the superseded no-small-single-use-helper.md rule. Committed in 70c85b38b.
• Test the rules against a representative source sample. PR #2306 applies them to exactly 20 handwritten source files and supplied five review counterexamples.
• Open a draft PR based on current origin/main. Opened PR #2305 from rules/structure-guidance.

Decisions and assumptions#

• The original submitted contents were the starting point. Later explicit user feedback authorizes the conditional rewrite, the unknown-shape rule, and the corrected truthiness filename.
• Unrelated root-worktree changes are out of scope and will remain untouched.

Implementation log#

• Created the branch from origin/main at aad216987.
• Committed the task specification before transferring the rule files.
• Left the root worktree unchanged, including all unrelated staged and unstaged changes.
• Renamed the truthiness rule to simplify-truthiness-checks.md without changing its bytes.
• Replaced the mechanical multiline-ternary rule with prefer-clear-conditionals: early returns remain preferred for exceptional branches, while value selection, shared work, positive predicates, mutation, and total vertical space decide whether a ternary should stay.
• Added validate-unknown-shapes: unknown values should cross a schema or domain guard instead of an anonymous chain of runtime checks.