Pin ruff lint rule selection explicitly

ruff.toml documented a rule set (F + E4/E7/E9) but never declared
select, so it inherited Ruff's implicit defaults. Those widened in
0.16 to include isort and pyupgrade, so CI (latest ruff) failed on
code that passed locally (0.15.18). Declaring select makes the lint
result identical across versions.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: ryanlefebvre1-maker

ruff.toml

@@ -1,5 +1,4 @@
 # Ruff configuration — STR Comping Agent
-# Uses Ruff's default lint rule set (pyflakes F + essential pycodestyle E4/E7/E9).
 line-length = 100
 target-version = "py310"
 
@@ -10,6 +9,14 @@ extend-exclude = [
     "_pre-sync-backup-*",
 ]
 
+[lint]
+# Pyflakes (F) + the essential pycodestyle checks (E4 imports, E7 statements,
+# E9 syntax/IO errors). Declared explicitly rather than relying on Ruff's
+# implicit defaults: those defaults widen between releases (0.16 added isort
+# and pyupgrade), which turns a routine `pip install ruff` into a lint failure
+# on unchanged code. Add rules here deliberately, not by upgrade.
+select = ["E4", "E7", "E9", "F"]
+
 [lint.per-file-ignores]
 # Tests insert the project root onto sys.path before importing local modules,
 # so their imports intentionally follow a non-import statement (E402).