chore: add .editorconfig formatting baseline (template-suite parity)

[devantler]

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What

Adds a stack-agnostic .editorconfig to platform-template, and brings two sample manifests that lacked a final newline into conformance with it.

Why

Template-suite parity is the lone open theme of the go-template roadmap epic #104 (reconciled as a portfolio-level holistic concern). Both Go-stack siblings ship an .editorconfig; platform-template had none:

Template	.editorconfig
go-template	✅
dotnet-template	✅
platform-template	❌ → ✅ (this PR)
gitops-tenant-template	❌ (held — KRO redesign in flux)

An .editorconfig gives everyone who clones the template a consistent formatting baseline (charset, EOL, final newline, trailing-whitespace, indent) regardless of editor — and via template-sync it propagates the baseline to every instance.

Design

platform-template is YAML / Markdown / JSON only (356 yaml + 33 md + 2 json; no Makefiles, shell, Go, or C#), so a 2-space space-indent default fits every file. The config mirrors the common base of the two siblings (charset=utf-8, end_of_line=lf, insert_final_newline, trim_trailing_whitespace) rather than introducing novel rules, keeping the suite consistent.

The repo already conformed almost completely — 0 trailing-whitespace files, 0 CRLF — so the only file changes needed were a final newline on two sample policy manifests (add-ns-quota, spread-pods-across-topology).

Validation

• kubectl kustomize builds clean for k8s/clusters/local, k8s/clusters/prod, and the cluster-policies base (an .editorconfig is editor-hint metadata; it cannot affect kustomize output).
• Re-scanned: 0 files missing a final newline, 0 trailing-whitespace, 0 CRLF after the change.

Additive and backward-compatible per the template's conservative-changes convention; template-owned (not in .templatesyncignore) so instances inherit it.