Summary:
This diff adds reusable pre-flight diagnostic checks and centralizes diagnosis
  merging at the framework level, so all benchmarks benefit automatically.

  **1. Shared check functions (`packages/common/diagnosis_utils.py`)**
  Added two reusable check functions to the shared diagnosis API:
  - `check_file_descriptor_limit()`: verifies ulimit is sufficient, with optional
    auto-fix that records the change under `notable_auto_fixes`
  - `check_selinux()`: detects SELinux Enforcing mode (causes HHVM segfaults)

  These join existing `check_ipv6_hostname()` and `check_port_available()`.

  **2. Generic pre-flight CLI (`packages/common/preflight_checks.py`)**
  A reusable script callable from any benchmark's bash runner:
  - Accepts `--benchmark <name>` and `--min-fds <N>` for per-benchmark config
  - Runs SELinux, IPv6 hostname, and file descriptor checks
  - Fixes inverted `check_ipv6_hostname` usage (returns True when IPv6 is
    detected, not necessarily broken — should not gate `all_ok`)

  **3. Framework-level diagnosis merge (`benchpress/cli/commands/run.py`)**
  After `job.run()`, `run.py` now automatically merges DiagnosisRecorder records
  (failures + auto-fixes) into metrics via `DIAGNOSIS_FILE_PATH`. Every benchmark
  gets diagnosis merging for free — no per-benchmark merge code needed. Removed
  the now-redundant `merge_failure_to_results()` call from tao_bench's
  `run_autoscale.py`.

  **4. MediaWiki integration**
  - `run.sh`: Calls common `preflight_checks.py` before benchmark; raises
    `ulimit -n` in bash so child processes (HHVM, nginx, wrk) inherit the limit
  - `jobs.yml`: Added `-U {auto_fix_ulimit}` arg with default `auto_fix_ulimit=1`
    to all 5 mediawiki job entries

Differential Revision: D98805269