Use Ubuntu template fallback when devcontainer is missing

[PabloZaiden]

Summary

• Default devbox creation to the Ubuntu template when no devcontainer definition is detected
• Wire the fallback through the CLI/core creation flow
• Update README guidance and test coverage for missing-devcontainer scenarios

Tests

• Added and updated core and examples tests covering the Ubuntu fallback behavior

[Copilot]

Pull request overview

This PR changes devbox’s workspace config resolution so that when no repo devcontainer is detected (and no prior template state exists), devbox automatically falls back to the built-in ubuntu template, and surfaces that decision to users via CLI output and documentation.

Changes:

• Add templateSelection metadata to ResolvedWorkspaceConfig to distinguish explicit/state/fallback template selection.
• Implement Ubuntu fallback behavior in resolveWorkspaceConfig, and print a CLI message when the fallback path is used.
• Update README and expand test coverage for missing-devcontainer and rebuild-without-state scenarios.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
tests/examples.test.ts	Updates example flow tests to validate automatic Ubuntu fallback messaging and state reuse behavior.
tests/examples.live.test.ts	Updates live example test to assert the fallback behavior/message without --template ubuntu.
tests/core.test.ts	Adds/updates unit tests for templateSelection and Ubuntu fallback when no devcontainer/state exists.
src/core.ts	Introduces templateSelection on ResolvedWorkspaceConfig and implements the Ubuntu fallback path in resolveWorkspaceConfig.
src/cli.ts	Prints a user-facing message when resolveWorkspaceConfig indicates the fallback template was used.
README.md	Documents the new automatic Ubuntu fallback behavior for up and rebuild <port>.

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