Publish branch-built GHCR images; make installer mirrorable and add PATH troubleshooting; update docs and changelog

[Widthdom]

Motivation

• Provide branch-built container images for AI/container workflows so users can run a branch-specific cdidx image from GHCR.
• Make the one-line installer usable in restricted/containerized environments by allowing mirrored GitHub endpoints and clearer failure guidance.
• Surface the installer and container changes in documentation and the changelog so users know how to pin, mirror, or run branch images.

Description

• Add a GitHub Actions workflow .github/workflows/container-branch.yml that builds and pushes branch and commit-tagged images to GHCR (ghcr.io/<owner>/cdidx:<branch> and :sha-<commit>).
• Add a multi-stage Dockerfile that publishes the cdidx tool from source and sets cdidx --version as the default command.
• Extend the one-line installer (install.sh) to accept CDIDX_GITHUB_API_URL and CDIDX_RELEASE_BASE_URL for mirror support and improve error messages with concrete fallback commands and checks; use these vars when resolving releases and downloading assets.
• Update README.md with PATH troubleshooting for ~/.local/bin, instructions for using mirrored endpoints when installing, and a new section documenting how to run branch-built GHCR images.
• Update CHANGELOG.md with entries describing the installer mirror support, PATH troubleshooting, and branch container publishing.

Testing

• No automated tests were added or run as part of this change; existing test suite and CI were not modified by these files.

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Codex Task