forgekit upgrade: update scaffolded project's CI/config to latest ForgeKit defaults

[SubhanshuMG]

Summary

Add a forgekit upgrade command that syncs a previously scaffolded project's CI workflows and config files to the latest ForgeKit defaults.

Motivation

When ForgeKit ships an improved GitHub Actions workflow or Dockerfile, existing projects have no way to pull those improvements in.

Proposed behavior

forgekit upgrade          # interactive diff + confirm
forgekit upgrade --dry-run  # show what would change, no writes
forgekit upgrade --force    # overwrite without prompting

Scope (v1)

• .github/workflows/ci.yml
• Dockerfile
• .eslintrc / eslint.config.js

Acceptance criteria

• Detects which template was used (reads .forgekit.json manifest)
• Shows a diff before writing
• --dry-run exits 0 with no writes
• Tested with unit + integration tests

[github-actions]

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