docs(readme): slim the README and add a contributing guide

[milanagm]

Brings the plugin README to the shape of the opencode plugin README. It keeps the
install path, the credentials and a short troubleshooting hint, and points to the
integration page for everything else. 240 lines down to 91.

Adds a CONTRIBUTING.md in the shape of the opencode one, with our own commands
for tests, ruff and the tag based marketplace release.

Worth knowing for review. The integration page still documents the manual hook
script path, so the CC_LANGFUSE_* option table, the env only trace tag and
parent trace variables, and the desktop @inline workaround are now documented
nowhere. A follow up docs PR should port them.

[claude]

Claude Code Review

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