Inline and harden tilde path expansion

[GrahamCampbell]

Remove the untildify dependency and replace it with an internal helper that fails closed when home paths cannot be safely expanded. Add coverage to ensure expansion failures happen before copy/download side effects.

[Copilot]

Pull request overview

This PR removes the external untildify dependency and replaces it with an internal lib/utils/untildify helper that performs strict (fail-closed) tilde expansion, with added unit coverage ensuring failures occur before any copy/download side effects.

Changes:

• Add internal lib/utils/untildify.js that expands ~, ~/..., and ~<current-user>/..., and throws controlled ServerlessErrors on unsafe/unavailable expansion.
• Switch create/download/local-template flows to use the internal helper instead of the untildify package.
• Add unit tests for untildify itself and for early-failure behavior in create/download flows.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
lib/utils/untildify.js	New internal tilde expansion helper with caching + controlled errors.
lib/utils/download-template-from-repo.js	Swap external untildify for internal helper for download target resolution.
lib/utils/create-from-local-template.js	Swap external untildify for internal helper for local template resolution.
lib/plugins/create/create.js	Swap external untildify for internal helper for --template-path and --path.
test/unit/lib/utils/untildify.test.js	New unit coverage for expansion behavior, caching, and error cases.
test/unit/lib/utils/download-template-from-repo.test.js	Add coverage for ~ download paths and early failure before side effects.
test/unit/lib/utils/create-from-local-template.test.js	Add coverage for ~ template paths and early failure before side effects.
test/unit/lib/plugins/create/create.test.js	Add coverage for ~ paths in create local template flow and early failure behavior.
package.json	Remove untildify dependency.

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