Renames and improves the CI analysis skill (formerly azdo-helix-failures).

Rename:

• Directory: azdo-helix-failures → ci-analysis
• Script: Get-HelixFailures.ps1 → Get-CIStatus.ps1
• Frontmatter name: azdo-helix-failures → ci-analysis
• The old name was too narrow — the skill does full CI analysis, not just failure reporting

SKILL.md improvements (skill-builder patterns):

• Critical rules — No PR approval/blocking (--approve/--request-changes), no branch switching (git checkout/gh pr checkout)
• Expanded trigger keywords — Added "why is CI red", "test failures", "retry CI", "rerun tests"
• Script Limitations section — Clarifies the script targets AzDO/Helix, but analysis patterns apply broadly
• Three Modes decision table — Distinguishes PR analysis vs Build ID vs Helix Job modes with -WorkItem optional
• Split "What the Script Does" — Per-mode sections with accurate step coverage (Build ID mode doesn't do Build Analysis or PR correlation)
• Inline anti-patterns — Warnings at key decision points:
  • Don't guess the mode — match input to mode
  • Canceled ≠ Failed — check Helix results before dismissing
  • Be cautious labeling failures as "infrastructure" — if Build Analysis didn't flag it, investigate before assuming transient
  • Missing packages on flow PRs — may be a code change, not feed infrastructure (e.g., CoreCLR vs Mono runtime pack)
• Heuristic trust warning — Don't blindly trust the script's recommendation over evidence
• gh pr checks tip — Valid --json fields documented (no conclusion — use state)

Script fixes:

• PowerShell 5.1 compatibility — Replaced em-dash characters with ASCII dashes, added UTF-8 BOM
• Cache directory — Renamed from helix-failures-cache to ci-analysis-cache

cc @dotnet/runtime-infrastructure