chore: adopt ruff as the linter, replace flake8

[shmulc8]

Stacked on the uv migration (The-PR-Agent#2509). Base is feature/uv-migration so this diff shows only the ruff changes; retarget to upstream main once The-PR-Agent#2509 merges.

What

Consolidates linting on ruff and, crucially, makes it enforced — ruff was already configured in pyproject.toml but never run in CI.

• Replace flake8 with ruff (pinned 0.15.20) in the dev group; enable the ruff pre-commit hook and drop the now-redundant isort hook (ruff sorts imports). The existing pre-commit.yml workflow now enforces linting on every PR.
• Apply ruff's safe autofixes only: remove unused imports (F401), collapse f-strings without placeholders (F541), sort imports (I001) — 302 fixes across 72 files. Also removed a duplicate GiteaProvider import.
• Defer pre-existing violations (E501, bare-except, bugbear rules, …) via an explicit lint.ignore list with a TODO to burn it down, so ruff runs green on adoption without a mass-churn reformat. F821 is scoped to pr_similar_issue.py (its optional deps are imported lazily).
• Update AGENTS.md to run ruff check instead of flake8.

Deliberately NOT included

ruff format (repo-wide formatting) — that's a large diff best done as its own PR plus a .git-blame-ignore-revs entry.

Verification

• ruff check → All checks passed; ruff pre-commit hook passes.
• PYTHONPATH=. uv run pytest tests/unittest → 1245 passed (autofixes broke nothing).

Follow-up debt (flagged, not fixed here)

• F811 — two real redefinitions worth a look: a shadowed handle_github_webhooks in bitbucket_app.py:311 and a re-imported Request in azuredevops_server_webhook.py:18.
• The lint.ignore list is the debt ledger — each entry is a rule to clean up and re-enable.

🤖 Generated with Claude Code

[shmulc8]

Superseded by The-PR-Agent#2511, opened natively in the upstream repo now that access is sorted.