Use native gh-aw skills and stdio MCP support in CI workflows

### Description

The MAUI CI scanner and related gh-aw workflows currently state that the gh-aw MCP Gateway does not support stdio MCP servers and therefore the `dotnet/arcade-skills` tooling is unavailable at agent runtime. That was true when the original scanner was authored, but it is no longer current.

Modern gh-aw supports:
- SHA-pinned external skills through top-level `skills:` frontmatter.
- Stdio MCP servers through `mcp-servers:` entries using `command` and `args`.
- MCP CLI exposure for mounted servers when appropriate.

`dotnet/arcade-skills` now provides the `ci-analysis` skill and the `dotnet-dnceng` plugin declares stdio servers for Helix, binlog analysis, and Maestro using `dotnet dnx`, plus the HTTP mihubot server. MAUI's in-repo `azdo-build-investigator` skill already expects to layer MAUI-specific guidance over `ci-analysis`.

Update the MAUI CI agent workflows to use supported native gh-aw integration rather than preserving the old REST-only limitation or manually installing floating global tools. The implementation should:

1. Add the required arcade-skills skill(s) using a full 40-character commit SHA.
2. Configure only the read-only stdio MCP servers that materially improve the scanner/fixer, with explicit tool allowlists.
3. Keep direct AzDO/Helix REST as a documented fallback rather than the primary path when higher-level tools are available.
4. Remove or correct statements claiming stdio MCP servers are unsupported.
5. Start any write-capable workflow changes in gh-aw staged mode and validate with `gh aw trial` before enabling writes.
6. Pin all external dependencies for reproducibility and supply-chain safety.
7. Confirm through a trial/audit that `ci-analysis` and at least the Helix tooling were actually invoked, not merely installed.

This should be a focused follow-up to the already-merged CI scanner work from #35734, not a resurrection of the obsolete #35329 implementation.

### Public API Changes

N/A. This changes repository automation only and introduces no .NET MAUI public API changes.

### Intended Use-Case

MAUI's scheduled CI scanner and fixer should use the maintained CI investigation logic from `dotnet/arcade-skills` while retaining MAUI-specific pipeline IDs, XHarness handling, failure deduplication, and merge-readiness rules from the in-repo guidance. This reduces duplicated prompt logic, removes stale platform assumptions, and lets the workflows benefit from improvements to `ci-analysis`, Helix investigation, and binlog analysis without relying on unpinned global tool installation.

Locate the MAUI CI agent workflows and the in-repo azdo-build-investigator skill, then compare their current gh-aw configuration with the merged scanner work from #35734. Use the gh-aw frontmatter and arcade-skills references in the issue as the starting point, run gh aw trial, and verify through its trial or audit output that ci-analysis and Helix tooling were invoked while writes remain staged.

Written by the indexing model from the issue text.