chore: adopt ruff linter (check-only) + fix two latent bugs it surfaced

[jramos]

What

Adopts ruff as a linter (check-only; the formatter stays deferred) — the ruff follow-up noted under the #106 disposition. Measured the tree first, then adopted only the high-value half.

Numbers (measured before adopting)

	ruff check (linter)	ruff format (formatter)
Footprint	134 findings, 79 auto-fixable	169/225 files, +10.8k/−4.5k lines
Nature	real issues (dead imports, empty f-strings, 2 latent bugs)	pure cosmetic reflow
Decision	adopt	defer

The #106 deferral blocker ("needs a line-length policy first") is real: the tree is written to ~100 chars (1208 lines > 88, but only 318 > 100). So E501 is intentionally not selected, and a one-shot line-length = 100 reformat (recorded in .git-blame-ignore-revs) remains the separate, optional path.

Two genuine latent bugs the linter caught

• evolution/tools/tool_module.py — Optional used in an annotation but never imported (F821), masked by from __future__ import annotations (the module imports fine, but it would break under get_type_hints() or if the future-import were removed). Fixed: added from typing import Optional.
• tests/validation/test_validator.py — a _ScriptedRunner was built then overridden by _PerTaskRunner and never used (F841): dead test setup. Removed.

Changes

• pyproject.toml — [tool.ruff.lint] select = ["E4","E7","E9","F"] (ruff's default set, no E501, no formatter); per-file-ignores "tests/**" = ["E402","E702","E741"] (tests may use compact import/statement style; all pyflakes F rules stay enforced in tests). ruff>=0.15,<0.16 pinned in the dev group.
• .pre-commit-config.yaml — ruff hook (--fix), pinned v0.15.20; ruff-format explicitly omitted.
• .github/workflows/tests.yml — a lint job running uv run ruff check evolution tests.
• Cleanups so shipped code (evolution/) is fully clean under the rules: autofixed 65 unused imports + 20 empty f-strings; preserved core/__init__.py re-exports via __all__; moved evolve_skill.py's basicConfig() below the import block (E402, no behavioral change); split semicolon statements in audit_gaming.py (E702).
• docs/upstream_pr_triage.md — recorded the adoption (review log + refreshed the fix(closed-loop): deliver the candidate skill when a task omits skills_src #106 row).

Verification

• ruff check evolution tests → clean (exit 0).
• Full non-slow suite green: 1754 passed.
• All 17 modified shipped modules import cleanly.
• Independent code-review pass: no findings.

Follow-up for the maintainer (not in this PR)

The lint CI job runs but isn't a required status check — making it required (and thus gating --auto merges) is a one-line branch-protection change. ruff-format + gitleaks (#107) remain deferred.