feat: add structured error codes (PLB-XXXX) with documentation links

[stephrobert]

Summary

Add structured error codes (PLB-XXXX) to every issue detected by Plumber, with direct links to dedicated documentation pages.

Changes

New file

• control/codes.go — Central error code registry with 14 codes (PLB-0101 to PLB-0501), ErrorCode type, ErrorCodeInfo struct, and lookup/listing functions

Modified files

• control/types.go — Added Code ErrorCode and DocURL string fields to issue structs
• cmd/analyze.go — Display [PLB-XXXX] prefix on each issue line + ↳ docs: link + Codes column in summary table
• control/mrcomment.go — Include error codes and doc links in MR comments
• 10 control files — Assign error codes to all issue creation points

Error Codes

Code	Issue	Control
PLB-0101	Forbidden image tag	containerImageMustNotUseForbiddenTags
PLB-0102	Image not pinned by digest	containerImageMustNotUseForbiddenTags
PLB-0103	Unauthorized image source	containerImageMustComeFromAuthorizedSources
PLB-0201	Branch not protected	branchMustBeProtected
PLB-0202	Non-compliant branch protection	branchMustBeProtected
PLB-0301	Hardcoded job	pipelineMustNotIncludeHardcodedJobs
PLB-0302	Outdated include version	includesMustBeUpToDate
PLB-0303	Forbidden include version	includesMustNotUseForbiddenVersions
PLB-0401	Required component missing	pipelineMustIncludeComponent
PLB-0402	Required component overridden	pipelineMustIncludeComponent
PLB-0403	Required template missing	pipelineMustIncludeTemplate
PLB-0404	Required template overridden	pipelineMustIncludeTemplate
PLB-0501	Debug trace enabled	pipelineMustNotEnableDebugTrace

CLI Output Example

[PLB-0101] Job "build" uses image "node:latest" with forbidden tag "latest"
  ↳ docs: https://getplumber.io/e/PLB-0101

Summary table now includes a Codes column with links to documentation.

Documentation

Companion PR for doc pages: getplumber/getplumber.io#77

Closes #92