feat: whoami, global --dry-run, colorized export, changelog, e2e test, contributing guide

[VelarisW58]

Six additions:

• mcpm whoami — shows detected AI clients, their active config paths, and installed servers per client
• Global --dry-run — mcpm --dry-run install/uninstall/import/sync ... previews client config writes without touching disk, and skips rollback snapshot creation
• Colorized mcpm export — JSON printed to an interactive terminal is now syntax-highlighted; piped/redirected output stays plain (still valid JSON for jq/shell redirects)
• mcpm changelog — generates a changelog grouped by conventional-commit prefix (feat/fix/refactor/test/docs/chore/ci) from git log since the last tag, with --from <ref> and --limit <n>
• End-to-end test (e2e-install-run-uninstall.test.ts) — exercises install → run → uninstall against a real temp config file, only mocking true external boundaries (registry, client detection, inquirer, child_process)
• CONTRIBUTING.md — extended the existing registry-focused guide with a code-contribution section (dev setup, test/lint workflow, adding a subcommand, the execFileSync-over-execSync security note)

Verified: tsc --noEmit, vitest run (58/58), eslint, manual smoke tests for whoami, --dry-run (confirmed no real file writes), export (piped output still valid JSON), and changelog (real output against this repo's history).