docs(templates): add canonical security matrix template

[ruromero]

Summary

Adds docs/templates/security-matrix.template.md — a canonical template capturing the required structure (sections, table columns, placeholder rows) shared by all security-matrix-*.md files.

• Defines all required sections: Version Stream, Supportability Matrix, Source Pinning Method, Ecosystem Mappings, Forward Pointer
• Supportability Matrix uses the exact column set: RHTPA Version | Build | Build Date | trustify | trustify-ui | Notes
• Ecosystem Mappings uses the exact column set: Ecosystem | Repository | Lock File | Check Command | Upstream Branch
• Includes inline HTML comments explaining each section's purpose
• Verified that existing matrices (2.1.x, 2.2.x, 3.0.x) conform to the template structure

Test plan

• Template renders correctly as GitHub-flavored Markdown
• All required sections present with correct heading levels
• Table columns match the exact specification
• Existing security-matrix files (2.1.x, 2.2.x, 3.0.x) conform to template structure
• Placeholder values used consistently for variable data

Refs: TC-5135

[ruromero]

Verification Report for TC-5135 (commit ae9d6cd)

Check	Result	Details
Review Feedback	N/A	No review comments on this PR
Root-Cause Investigation	N/A	No sub-tasks created; nothing to investigate
Scope Containment	PASS	PR touches exactly the file specified in the task: docs/templates/security-matrix.template.md
Diff Size	PASS	49 additions across 1 new file; proportionate for a template document
Commit Traceability	PASS	Single commit references TC-5135 via "Refs: TC-5135"
Sensitive Patterns	PASS	No secrets, credentials, or sensitive patterns in added lines
CI Status	PASS	Plugin Validation and Skill Lint both pass
Acceptance Criteria	PASS	6 of 6 criteria met; template has correct structure and all 3 existing matrices conform
Test Quality	N/A	Repetitive Tests: N/A, Test Documentation: N/A, Eval Quality: N/A
Test Change Classification	N/A	No test files in this PR
Verification Commands	N/A	No verification commands specified

Overall: PASS

All checks pass. The template is correctly structured, uses consistent placeholders, and the three existing security matrix files (2.1.x, 2.2.x, 3.0.x) conform to the template structure.

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This comment was AI-generated by sdlc-workflow/verify-pr v0.12.3.