Write a 'writing a custom detector' contributor guide #3

Context

taintline detectors implement a tiny Detector Protocol (name + a pure run(trace) -> list[Finding]). Everything needed to write one is in the code, but there is no contributor-facing guide walking through it. This is the single highest-leverage doc for attracting agent-safety contributors.

What to do

Add docs/writing-a-detector.md covering:

• the Detector Protocol and the purity/determinism contract (no LLM, no network, no clock, no RNG);
• the Trace/Span/Finding/Verdict types a detector works with;
• a minimal worked example (e.g. flag any tool span with status == "ERROR"), including how to register it in ALL and add a seeded fixture + test;
• how --fail-on gates the exit code vs. what always runs.

Link it from the README and CONTRIBUTING.md.

Acceptance criteria

• docs/writing-a-detector.md exists and a reader can implement + register a new detector from it alone.
• The worked example is real, runnable code (copy-pasteable).
• README and CONTRIBUTING.md link to it.

Pointers

• src/taintline/model.py:90 — the Detector Protocol; Finding/Verdict above it
• src/taintline/detectors/reliability.py:123 — SwallowedErrorDetector, a compact example to model the guide on
• src/taintline/detectors/__init__.py — registration in ALL
• docs/CONTRACT.md — the frozen type contract to reference