docs: add opt-in unpackaged WinUI run guidance

[Nikola Metulev (nmetulev)]

Description

Add concise guidance for running WinUI apps unpackaged only when requested or needed for targeted debugging; packaged remains the default.

• Show the project setting and direct winapp run --debug-output command.
• Note runtime and package-identity requirements, Sandbox debug-capture limitations, and restoring packaged mode after debugging.
• Align the migration rule with the same opt-in workflow and add a short 0x80073D54 troubleshooting entry.

Only three files change against staging; no versions, release policy, scripts, or tools change.

Related Issue

Related to microsoft/winappCli#888 (separate CLI diagnostic-hint improvement; not fixed by this PR).

Type of Change

• 📝 Documentation

Affected area

• Skill: winui-dev-workflow, winui-wpf-migration
• Repo-level docs / governance: Unreleased changelog entry

Checklist

• Tested locally on Windows
• SKILL.md frontmatter remains valid; cross-references resolve
• Added a bullet to ## [Unreleased] in CHANGELOG.md
• No version fields changed

Screenshots / Demo

Not applicable to the documentation changes.

Additional Notes

Validation used an isolated Daylight copy on ARM64 Windows with WinApp CLI 0.6.3-nmetulev-devtools-extraction.81 (development build):

• Reproduced a package-identity-dependent startup failure locally and in Sandbox, confirming it is not Sandbox-specific.
• After adapting the test copy's storage path, verified local unpackaged startup, persistence across restart, and packaged compatibility; 53 UI checks passed.
• The same fixed source launched unpackaged in Sandbox, with native no-package-identity confirmation and data-file creation. Further interaction, screenshots, and save/reload validation were blocked by sandbox_target_ambiguous; full Sandbox validation is not claimed.
• Both edited skills passed skills-ref validate; git diff --check passed.

App changes and test artifacts are not part of this PR.