Ship the listener inside the package, allow no-port URLs

[timdegroot1996]

Summary

Closes #317.

• Moves the Robot Framework listener into robotframework_dashboard/robotdashboardlistener.py so it's available the moment the package is pip installed — no separate download from GitHub, which matters in closed-off/offline CI environments. Reference it via the dotted module path: --listener robotframework_dashboard.robotdashboardlistener.
• Drops the requests dependency entirely: HTTP (multipart upload, JSON DELETE, HTTPS/SSL context, basic auth) is now implemented on urllib/ssl/json from the standard library, so the packaged listener adds zero runtime dependencies beyond Robot Framework itself.
• Fixes the "no port" oversight raised in the issue: host/port listener args (and --host/--port on the standalone robotdashboardscript.py pusher) can omit the port entirely — pass port= (listener) or --port "" (script) — for servers reachable on the protocol's default port (80/443) or behind a reverse proxy that doesn't expose one.
• example/listener/robotdashboardlistener.py is now a one-line re-export of the packaged module, kept for anyone who prefers pointing --listener at a local file path instead of the dotted import.
• Docs and code now cross-link both ways (docs page ↔ canonical source file ↔ example re-export) so the implementation is easy to find from either side.

Test plan

• bash scripts/python-tests.sh — 361/361 passing, robotdashboardlistener.py at 100% coverage (50 new tests mocking urlopen instead of requests)
• Manual end-to-end smoke test against a real running dashboard server (not mocked): ran real Robot Framework suites through robot --listener robotframework_dashboard.robotdashboardlistener via the actually-pip installed package, confirming output upload, log upload + DB path linking, and the no-port path (server bound to port 80, listener with port=) all work over real HTTP with zero extra dependencies installed
• npm run docs:build — docs build clean
• Updated docs/listener-integration.md, docs/installation-version-info.md, .github/skills/listener-integration.md

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