chore: integrate pre-push vulture dead-code scanner

[seroperson]

Closes #1252

Summary

• Added vulture==2.16 to dev deps and configured it in pyproject.toml. paths covers production tree only (gittensor, neurons, scripts); tests/ is omitted so test-only-API symbols (prod functions whose sole references live under tests/) surface as findings.
• Excluded files whose schema mirrors an external wire format and shouldn't be pruned by static analysis: gittensor/validator/issue_competitions/contract_client.py (on-chain shape) and gittensor/utils/mirror/models.py (das-github-mirror response shape).
• Framework false positives suppressed via [tool.vulture] ignore_decorators (@click.command/@click.group/@*.command/@*.group) and ignore_names
• Hook: added vulture under stages: [pre-push] in .pre-commit-config.yaml. CI already runs SKIP=pytest pre-commit run --all-files --hook-stage pre-push, so the existing lint job picks it up. Renamed the step from "Run pyright" to "Run pre-push checks (pyright, vulture)" for accuracy.

Deletions surfaced by the scanner

• gittensor/classes.py - FileChange.from_github_response, PullRequest.set_file_changes, PullRequest.current_labels, PullRequest.label_timeline_order, unused DefaultDict import.
• gittensor/utils/utils.py - parse_repo_name, unused Dict import.
• gittensor/utils/github_api_tools.py - get_github_id, a 3-line wrapper around get_github_identity(token).github_id. Only the retry test called it; that test now exercises get_github_identity directly and the retry coverage is preserved.
• gittensor/validator/utils/datetime_utils.py - parse_github_timestamp_to_cst, CHICAGO_TZ, unused pytz import. All call sites disappeared with refactor: strip legacy scoring pipeline; mirror-only master repos #1202's strip of the legacy GraphQL constructors.
• gittensor/validator/pat_storage.py - remove_pat. CRUD primitive with no prod caller; orphaned scaffolding for a deregister-cleanup path that was never wired up. TestRemovePat (3 tests) went with it.
• gittensor/validator/storage/repository.py - refresh_stale_pr_states method, and the REFRESH_STALE_PR_STATES SQL constant in queries.py it consumed. The lone test reference was a defensive mock_repo.refresh_stale_pr_states.return_value = 0 setup, not a real test.
• gittensor/constants.py - MAX_FILES_PER_GRAPHQL_BATCH.
• neurons/base/validator.py - self.lock = asyncio.Lock(). Bittensor-template carry-over; never had an async with guard in this repo's history.
• tests/validator/conftest.py - gutted to PRBuilder.create() only (the lone fixture/import test_pioneer_dividend.py actually consumes). pr_factory/issue_factory fixtures, IssueBuilder and its completed/transferred/not_planned/no_reason methods, MinerScenario, and all six miner-scenario fixtures - orphaned by refactor: strip legacy scoring pipeline; mirror-only master repos #1202.
• tests/cli/test_cli_helpers.py - removed dead _Sentinel guard + exec_was_called plumbing in test_register_aborts_on_github_503_before_contract_call. The sentinel was defined but never patched into the test, so its "must not reach exec" assertion was vacuously true.
• Renamed unused unpack/lambda vars to _-prefixed in gittensor/cli/issue_commands/helpers.py (_sign, _digits, exponent) and two test lambdas.

Testing

• Tests added/updated
• Manually tested

Test plan

uv run vulture                                           # exit 0, no findings
uv run ruff check                                        # all checks passed
uv run ruff format --check                               # 128 files already formatted
uv run pyright                                           # 0 errors, 0 warnings
uv run pytest tests/ -q                                  # 722 passed
SKIP=pytest uv run pre-commit run --all-files --hook-stage pre-push  # all hooks pass

How it looks like