Move best-practices index to BESTPRACTICES.md and document CONTEXT.md pattern

Aligns index files with context-load discovery: BESTPRACTICES.md and CONTEXT.md
now follow the same thin-index pattern as MEMORY.md, auto-loaded at session start.
Adds documentation for both new index types to CLAUDE.md Documentation Standards.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

best-practices/INDEX.md

@@ -1,18 +0,0 @@
-# Best Practices Index
-
-Generalised best practices extracted from real project work. Each topic file is self-contained — read only the files relevant to the current project.
-
-## Topics
-
-- [Validation & Deployment](validation.md) — Validate locally, deploy once; full-chain testing; pre-flight checks
-- [Secrets Management](secrets-management.md) — SOPS + age, credential handling, file naming, encryption gotchas
-- [Git & Source Control](git-source-control.md) — Commit practices, GitOps workflows, remote conventions
-- [Kubernetes Patterns](kubernetes.md) — Volume mounts, deployment strategies, naming, bootstrap ordering
-- [Helm Charts](helm.md) — Schema validation, version verification, values structure
-- [Ansible](ansible.md) — Inventory, templates, idempotency, credential safety
-- [Scripting](scripting.md) — Shell conventions, verification scripts, idempotency, colour output
-- [Documentation Standards](documentation.md) — CLAUDE.md, MEMORY.md, FUTURE.md, README.md structure and tiered memory
-- [Milestones & Reflections](milestones.md) — Milestone workflow, verification, reflection process
-- [Debugging Methodology](debugging.md) — Systematic diagnosis, full-chain testing, common pitfalls
-- [Claude Code Skills](skills-development.md) — Skill authoring, context injection, tool restrictions
-- [Linting & Formatting](linting.md) — Tool choices per language, PostToolUse hook, pre-commit integration, formatter contract