chore: remove redundant root-level community health files

[wcmchenry3-stack]

Context

The root of the .github repo contains older, diverged copies of community health files that duplicate canonical versions in community-health/:

Root file	Canonical file	Status
/SECURITY.md	community-health/SECURITY.md	Root is 30-line stub; canonical is 104 lines
/PULL_REQUEST_TEMPLATE.md	community-health/PULL_REQUEST_TEMPLATE.md	Root missing Mobile checklist section
/ISSUE_TEMPLATE/ (2 files)	community-health/ISSUE_TEMPLATE/	Duplicates

Since sync-community-health.yml explicitly pushes from community-health/ to all org repos, the root copies serve no purpose and create confusion about which is authoritative.

Acceptance Criteria

• /SECURITY.md deleted from repo root
• /PULL_REQUEST_TEMPLATE.md deleted from repo root
• /ISSUE_TEMPLATE/ directory and all contents deleted from repo root
• community-health/ directory is the single source of truth for all community health files
• Verified that sync-community-health.yml FILE_MAP references only community-health/ paths (not root paths)
• No broken references in any workflow file to the deleted paths

Files to Delete

/SECURITY.md
/PULL_REQUEST_TEMPLATE.md
/ISSUE_TEMPLATE/bug_report.md
/ISSUE_TEMPLATE/feature_request.md

Dependencies

None — can be worked in parallel with all other Phase 2 issues.

Part of Epic

#77