Add CONTRIBUTING, CHANGELOG and issue templates

.github/ISSUE_TEMPLATE/bug_report.md

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+---
+name: Bug report
+about: Something returns a wrong answer or crashes
+labels: bug
+---
+
+**Command**
+
+```
+netprobe ...
+```
+
+**What happened**
+
+Paste the output. If it is a crash, include the whole traceback.
+
+**What you expected instead**
+
+For a wrong-answer bug, say what the correct value is and why - a reference
+(RFC, vendor doc, another tool's output) helps a lot.
+
+**Environment**
+
+- `netprobe --version`:
+- `python --version`:
+- OS:

.github/ISSUE_TEMPLATE/feature_request.md

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+---
+name: Feature request
+about: Propose a new command or option
+labels: enhancement
+---
+
+**The diagnostic question you are trying to answer**
+
+Describe the situation, not the flag. "I need to know which of these 40 hosts
+answers on 8080" is more useful than "add a --parallel option".
+
+**How you solve it today**
+
+Which tool, and what is awkward about it.
+
+**Would this need a dependency?**
+
+The project is standard-library only. If the feature needs something from PyPI,
+say what and why nothing in the stdlib covers it.

.github/pull_request_template.md

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+**What changed**
+
+**Why**
+
+If you made a non-obvious choice, explain the reasoning here - that is the part
+nobody can reconstruct from the diff later.
+
+**Checklist**
+
+- [ ] `python -m unittest discover -s tests` passes
+- [ ] `ruff check .` is clean
+- [ ] New behaviour has tests, and boundary cases are covered
+- [ ] Exit status follows the convention (`0` ok, `1` check failed, `2` usage)
+- [ ] README updated if a user-visible flag or output changed

CHANGELOG.md

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+# Changelog
+
+All notable changes to this project are documented here. The format follows
+[Keep a Changelog](https://keepachangelog.com/en/1.1.0/) and the project uses
+[Semantic Versioning](https://semver.org/spec/v2.0.0.html).
+
+## [Unreleased]
+
+## [0.1.0]
+
+First release. Six commands, no dependencies.
+
+### Added
+
+- `subnet` - CIDR description, splitting, membership tests and summarisation,
+  with correct `/31` (RFC 3021 point-to-point) and `/32` host counts
+- `scan` - threaded TCP port sweep over a host or a whole CIDR block, with
+  flexible port specs (`22`, `22,80`, `1-1024`) and service annotation
+- `latency` - TCP connect timing with min/avg/p95/max, packet loss and jitter
+- `dns` - forward, reverse and round-trip resolution through `getaddrinfo`
+- `http` - endpoint health with hop-by-hop redirect chain walking
+- `mac` - MAC normalisation across notations, OUI vendor lookup, and decoding
+  of the locally-administered and multicast flag bits
+- Global `--json` flag on every command
+- Optional `netprobe.ini` for per-command defaults, applied as argparse
+  defaults so explicit flags always win
+- 84 tests and a CI matrix over ubuntu/windows/macos on Python 3.9 and 3.12
+
+[Unreleased]: https://github.com/saeed205/netprobe-cli/compare/v0.1.0...HEAD
+[0.1.0]: https://github.com/saeed205/netprobe-cli/releases/tag/v0.1.0

CONTRIBUTING.md

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+# Contributing
+
+## Setup
+
+No dependencies to install for the library or the tests:
+
+```bash
+git clone https://github.com/saeed205/netprobe-cli
+cd netprobe-cli
+python -m unittest discover -s tests -v
+```
+
+Only the linter needs installing:
+
+```bash
+pip install ruff
+ruff check .
+```
+
+## Adding a command
+
+Commands self-register, so nothing in the argument parser needs editing.
+
+1. Create `netprobe/yourcommand.py`.
+2. Write the logic as **plain functions that take and return data** - no
+   printing, no `argparse` types in the signature. That is what makes it
+   testable without a network.
+3. Add a `_handle(args)` that calls those functions and hands the rows to
+   `output.emit()`.
+4. Register the parser:
+
+   ```python
+   from .cli import register
+
+   @register
+   def _add_parser(subparsers):
+       p = subparsers.add_parser("yourcommand", help="one line of help")
+       p.add_argument("target")
+       p.set_defaults(handler=_handle)
+   ```
+
+5. Import the module in `netprobe/commands.py` and add it to `_MODULES`,
+   keeping both alphabetical.
+
+## House rules
+
+- **Standard library only.** A diagnostics tool that cannot be installed on a
+  locked-down jump box is not much use. A dependency needs a strong argument.
+- **Python 3.9 is the floor.** Typing-module spellings (`Dict`, `Optional`)
+  are used deliberately - see the comment in `ruff.toml` before "modernising"
+  them.
+- **Exit status is an interface.** `0` success, `1` the check failed, `2`
+  usage error. People put these commands in monitoring checks; do not return
+  `0` for a failed probe.
+- **Never print from a helper.** Rendering belongs in `output.py` so that
+  `--json` keeps working everywhere for free.
+
+## Tests
+
+Plain `unittest`. Anything that opens a socket does not belong in the suite -
+CI runs on sandboxed runners with no egress. Test the parsing, the arithmetic
+and the precedence rules; those are where the bugs actually live.
+
+Worth testing explicitly: boundary values (`/31`, `/32`, empty input, single
+element), and anything where a plausible-looking implementation gives a wrong
+answer.
+
+## Pull requests
+
+Say what changed and why. If you made a non-obvious choice, put the reasoning
+in the description - that is the part nobody can reconstruct from the diff six
+months later.