Enforce Go best practices (task 344)

- Magefile.go: add Default() target so bare `mage` builds the binary;
  fix Install()/Uninstall() to use $GOPATH/bin (default ~/go/bin) instead
  of the previous hardcoded ~/.local/bin path; use cp -v for visibility;
  fix 0755 -> 0o755 octal literal in createBinDir; extract binaryName const
- cmd/geheim/main.go: add -version flag (prints version.Version and exits);
  pass flag.Args() instead of os.Args[1:] so flags are parsed cleanly
- internal/cli/cli.go: remove dead fatal() and prompt() helpers that were
  never called anywhere in the codebase

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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