docs: Write 'writing a custom detector' contributor guide (#3) - #17

Closes #3

Summary

Adds a comprehensive contributor guide docs/writing-a-detector.md detailing how to design, implement, register, and test a custom deterministic detector in taintline.

What's Included

• Detector Protocol & Purity Contract: Explains the Detector protocol (name + run(trace) -> list[Finding]) and zero-LLM / zero-I/O / deterministic constraints (no network, no system clock, no RNG, no un-sorted set iteration).
• Core Types Explained: Documented Span, Trace (tree navigation helpers root(), in_order(), children()), Finding, and Verdict.
• Copy-Pasteable Worked Example: Complete step-by-step walkthrough creating an ErrorStatusDetector class, registering it in ALL (src/taintline/detectors/__init__.py), creating a seeded JSON fixture (tests/fixtures/seeded/), and writing unit/acceptance tests.
• CI Exit Code Gating: Clear explanation of how --fail-on gates the CLI exit code (0 vs 1) independently of finding reporting.
• Cross-Linked: Linked cleanly from README.md under Detectors and CONTRIBUTING.md.

Verification

• Markdown rendered cleanly; links validated.
• pytest: 49/49 passed (1.10s).
• ruff check: clean (0 issues).