refactor(db): extract shared DB module from duplicated patterns

Create tooling/db/common.py with resolve_db_path(), load_config(),
get_connection(), and ensure_venv(). Update ticket, sprint,
sqlite_connector.py, and decisions_sync.py to import from common
instead of duplicating. Fixes pre-existing NameError in
decisions_sync.py (missing import os). Add pyproject.toml with
ruff config (ruff==0.15.9, CVE-clean) and dev dependencies.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

pyproject.toml

@@ -0,0 +1,26 @@
+[project]
+name = "settled-reach-tooling"
+version = "0.1.0"
+requires-python = ">=3.11"
+dependencies = [
+    "PyYAML",
+    "jsonschema",
+    "numpy",
+]
+
+[project.optional-dependencies]
+dev = [
+    # ruff 0.15.9 — checked clean against NVD + OSV, no CVEs on record (2026-04-05)
+    "ruff==0.15.9",
+]
+
+[tool.ruff]
+line-length = 100
+target-version = "py311"
+
+[tool.ruff.lint]
+# E9xx: Runtime syntax/encoding errors
+# F401: Unused imports
+# F811: Redefinition of unused name
+# F821: Undefined name (catches missing imports like bare `os`)
+select = ["E9", "F401", "F811", "F821"]