docs: standardize CONTRIBUTING against repo-standards

[retr0h]

Standardizes this repository's CONTRIBUTING.md against the repo-standards capability, which now fixes the middle sections as well as the opening and closing ones (osapi-io/specs#89, osapi-io/specs#90).

The problem

The five Go repositories stated the same conventions five different ways:

	gohai	nats-client	nats-server	orchestrator	osapi
Function signatures	missing	Code style	Code style	Code standards	pointer only
Go patterns	under Testing	Code style	Code style	Code standards	pointer only
Code standards	✓	✗	✗	✓	✓
Casing	Title Case	sentence	sentence	Title Case	—

A reader comparing two repositories could not tell whether a difference in the document meant a difference in the rule.

Section order, now fixed

Before you start → Prerequisites → Setup → Project structure →
Code style → Code standards → Testing → (repo-specific) →
Before committing → Branching → Commit messages → Submitting a PR → AI usage → FAQ

Anything invented for one repository now sits after Testing.

The shared text is byte-identical

The Code standards body and the Test file conventions list are the same bytes in all five repositories — verified by hash. A difference in wording now means a difference in rule.

The linter list is gone

Every repository listed "errcheck, errname, goimports, govet, prealloc, predeclared, revive, staticcheck". Every .golangci.yml enables unused — named in no prose — and configures goimports under formatters:, not among the linters. Five identical copies of a wrong summary of the file beside them. The guide now names .golangci.yml instead.

Verification

just test passes; coverage unchanged.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

@@           Coverage Diff           @@
##             main     #452   +/-   ##
=======================================
  Coverage   99.94%   99.94%           
=======================================
  Files         484      484           
  Lines       24935    24935           
=======================================
  Hits        24921    24921           
  Misses         14       14           

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