Add CONTRIBUTING, CHANGELOG and issue templates

[saeed205]

Writes down the conventions that so far only existed as a pattern in the code.

CONTRIBUTING.md documents the five steps to add a command, and the four house rules that are not obvious from reading a single module:

• standard library only - a diagnostics tool you cannot install on a locked-down jump box is not much use
• 3.9 is the floor, and the typing-module spellings are deliberate (points at the ruff.toml comment before anyone "modernises" them)
• exit status is an interface - people put these in monitoring checks, so a failed probe must not return 0
• helpers never print; rendering stays in output.py so --json keeps working everywhere for free

The testing section states the constraint directly: anything that opens a socket does not belong in the suite, because CI runs on sandboxed runners with no egress.

CHANGELOG.md in Keep a Changelog format, with 0.1.0 covering the six commands.

Issue templates ask for the diagnostic question rather than the requested flag, and the bug template asks for the correct value and a reference - for a wrong-answer bug in subnet math, that is the whole report.