Add project name namespacing for extended images

[janderland]

Summary

Resolves image collision issues when multiple projects use fenv as a submodule by namespacing extended images with the project name.

Changes

• Image naming: Extended images are now named fenv-ext-{project}:{tag} instead of fenv-ext:{tag}
• Auto-detection: Project name is automatically derived from:
  1. FENV_PROJECT_NAME environment variable (if set)
  2. Git remote repository name
  3. Current directory name (fallback)
• GitHub Actions: Added optional project_name input to the action
• Documentation: Updated README with new configuration options

Problem Solved

Before this change, if two projects (projA and projB) both used fenv as a submodule, they would share the same extended image name fenv-ext:{tag}. This caused issues:

• If both projects were at the same git commit hash, they'd have the same tag but potentially different Dockerfiles
• Image caches would collide and overwrite each other
• One project's extended image could incorrectly be used by another project

Test Plan

• Verified fenv.sh builds extended images with project name
• Confirmed project name derivation logic works correctly
• Test in a real project using fenv as a submodule
• Verify GitHub Actions workflow with the new input

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