Bootstrap temporary Composer for local project creation

[Copilot]

What changed?

• Added a verified, command-scoped Composer resolver for local project creation.
  • Uses composer from PATH when available.
  • Otherwise downloads and verifies Composer’s official installer, creates a temporary PHAR, and removes all artifacts on exit.
• Executes temporary Composer through the resolved PHP binary.
• Allows project create preflight without global Composer; PHP compatibility remains required.
• Preserves Docker and existing global-Composer behavior.

<selected-php> <temporary-dir>/composer.phar install --no-interaction

Why?

Local project creation failed on hosts with PHP but no globally available Composer. The fallback avoids global installation and does not persist Composer artifacts in the project or user directories.

How was this tested?

• Added resolver tests for global resolution, verified bootstrap, checksum/status failures, cancellation, cleanup, and PHP/PHAR invocation.
• Ran go test ./... and go vet ./internal/system ./cmd/project.

Related issue or discussion

• Fixes project create: bootstrap Composer temporarily when it is not installed globally #1304