chore(rmi-backend,audit): delete ruff.toml — pyproject.toml [tool.ruff] is canonical

Phase 1 of AUDIT-2026-Q3.md item P1.8.

ruff.toml was overriding pyproject.toml [tool.ruff] silently:
  ruff.toml:    line-length 120, ignored SIM102/RUF006/B008
  pyproject.toml: line-length 100, security rules enabled (canonical)

Two configs contradicting each other silently is the worst-case
configuration — neither developer nor CI notice, and the strict
[tool.ruff] security rules (S) never run.

Deleted ruff.toml. Added defensive .gitignore entry. ruff now reads
only pyproject.toml. Verified with ruff check --show-settings:
  linter.line_length = 100
  linter.pycodestyle.max_line_length = 100

ruff.toml moved to /tmp/rmi-backend-archive-2026-07/ruff.toml.removed-2026-07-06.

.gitignore

@@ -88,3 +88,4 @@ data/papers/
 *.h5
 *.hdf5
 main.py.bak
+ruff.toml

ruff.toml

@@ -1,17 +0,0 @@
-# RMI Backend - Strict linting config
-target-version = "py311"
-line-length = 120
-
-[lint]
-select = [
-    "E", "F", "I", "W", "UP", "B", "C4", "SIM", "RUF",
-]
-ignore = [
-    "E501",   # Line too long
-    "RUF006", # create_task result unused (fire-and-forget patterns)
-    "SIM102", # Nested if — existing code convention
-    "B008",   # function-call-in-default (FastAPI Depends pattern, intentional)
-]
-
-[lint.per-file-ignores]
-"app/core/lifespan.py" = ["T20"]   # print() for startup logging is intentional