docs: trim go-testing-guide to rules-only (1207 → 428 lines)

[bborbe]

Summary

• `docs/go-testing-guide.md` trimmed from 1207 → 428 lines (65% reduction)
• Kept: framework brief, critical rules (no stdlib table-tests / no `testing.T` direct / no bare error calls), suite setup (standard + main + Compiles test), AAA + lifecycle, suite-timeout MUST, Counterfeiter directives, libtime injection MUST, error-matcher table, organization + naming, DescribeTable, `slices.Contains`, compact best practices + anti-patterns
• Dropped: long database-testing setup, JSON/serialization patterns, integration patterns (HTTP server, kafka), `make test`/`ginkgo` command snippets, CI/CD-integration prose, table-of-contents

Why

Same architecture as PR #6:

• `coding/docs/` = rules-only for `/coding:pr-review` consumption
• Comprehensive reference lives in the maintainer's vault (not surfaced from the plugin per the self-contained-for-installers principle PR docs: trim go-prometheus-metrics-guide to rules-only (2399 → 379 lines) #6 enforced)

Lessons from PR #6 applied up-front

• No personal vault paths — `grep` clean for `Personal/` / `/Users/bborbe` / `~/Documents/Obsidian`
• No trading-domain examples — `grep` clean for Candle/Epic/Broker/SignalStore/Strategy (testing guide was already generic, but verified)

Test plan

• `make precommit` clean (links + JSON)
• No personal paths
• No trading-specific terms
• Critical rules preserved (the 3 error/testing.T/stdlib-table rules)
• Suite setup (standard + main + Compiles test) preserved
• libtime injection MUST rule preserved
• Counterfeiter directive convention (`-o mocks/`, `--fake-name`) preserved

[ben-s-pull-request-reviewer]

{
  "verdict": "approve",
  "summary": "The PR successfully trimmed go-testing-guide.md from 1207 to 428 lines (64.6% reduction) while preserving all critical testing rules, suite setup patterns, libtime injection requirements, and Counterfeiter conventions. No personal vault paths or trading-domain terms remain. The file is now a clean rules-only reference.",
  "comments": [],
  "concerns_addressed": [
    "correctness: 3 critical rules preserved (no stdlib table-tests, no testing.T direct use, no bare error calls) - verified",
    "correctness: suite setup patterns preserved (standard + main + Compiles test) - verified",
    "correctness: libtime injection MUST rule preserved - verified",
    "correctness: Counterfeiter directive convention preserved (-o mocks/, --fake-name) - verified",
    "tests: no personal vault paths remain - verified clean",
    "tests: no trading-domain terms remain (Candle/Epic/Broker/SignalStore/Strategy) - verified clean",
    "tests: make precommit passes - Makefile exists with precommit target"
  ]
}