Polish CLI help consistency for commands, defaults, and stdin usage

[Copilot]

This PR addresses CLI usability/consistency gaps in command help text, defaults wording, and examples/guidance. The goal is to make the CLI easier to scan and less ambiguous without changing command behavior.

• Help text normalization

  • Simplified/standardized choice-option help phrasing to remove verbose, inconsistent wording.
  • Kept concise expansions where abbreviations are non-obvious:
    • --type: er, ba, ws
    • --algorithm: label_prop

• Stdin guidance consistency across commands

  • Standardized input help text to a canonical form:
    • Input network file (use '-' for stdin)
  • Applied consistently to file-input commands (including load and query) so piping expectations are clear.

• Top-level command discoverability

  • Improved subparser metadata for faster command discovery:
    • explicit commands section
    • consistent metavariable naming (COMMAND)
    • clearer invocation hint in help text (py3plex COMMAND --help)

• Parser clarity

  • Made --input-format destination explicit (dest="input_format") where used, aligning CLI definition with internal argument access.

• Focused ergonomics test updates

  • Tightened CLI ergonomics assertions to enforce canonical stdin help phrasing.
  • Added coverage for removal of redundant help wording patterns.

Example of the standardized help style now used:

$ py3plex centrality --help
...
positional arguments:
  input                 Input network file (use '-' for stdin)

options:
  --measure {degree,betweenness,closeness,eigenvector,pagerank}
                        Centrality measure (default: degree): degree, betweenness, closeness, eigenvector, or pagerank

https://github.com/user-attachments/assets/ed1693ec-c357-49bb-9831-6eb6ba370f7d

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