feat: Add namespaced skill deployment and harden Windows test/auth behavior

[shreejaykurhade]

Description

Adds support for optional namespace values in apm.yml and applies them consistently to deployed skills. Issue #739

When a package declares a namespace, deployed skill directories are prefixed as <namespace>.<skill-name>. This applies to native SKILL.md packages, promoted .apm/skills/* sub-skills, root project-local .apm/skills/*, and sync/orphan cleanup logic.

This change also improves Windows-related behavior in tests and authentication paths by fixing local CLI discovery, enforcing UTF-8 subprocess output decoding, removing unsupported downloader.github_host test assignments, cleaning unused imports, and tightening credential and path handling.

Documentation has been updated to describe the new namespace field in the README, manifest schema, and skills guide.

Fixes #

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Type of change

• Bug fix
• New feature
• Documentation
• Maintenance / refactor

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Testing

• Tested locally
• All existing tests pass
• Added tests for new functionality (where applicable)

Test commands

uv run pytest tests/unit/test_apm_package.py tests/unit/test_local_content_install.py tests/unit/integration/test_skill_integrator.py tests/integration/test_local_install.py tests/test_github_downloader.py -q
# Result: 324 passed, 2 skipped

uv run pytest tests/unit -q
# Result: 5311 passed, 1 warning

#Docs validation
npm ci
npm run build
# Result: docs build completed successfully and internal links are valid

[shreejaykurhade]

Please review