Remove the base directory and organize abstract templates by concern

[braboj]

Problem

templates/base/ originally held reusable abstractions. It now groups unrelated domains, while backend and frontend templates also act as abstractions. The folder no longer explains ownership clearly.

Proposed change

Promote the six existing concern directories and retain the other layers:

templates/
  core/
  language/
  security/
  workflow/
  infra/
  data/
  backend/
  frontend/
  platform/
  stack/
  manifest.yaml
  INTERVIEW.md

Folders identify concerns. Manifest dependencies determine inclusion; the explicit core: list determines the always-included tier. Preserve existing template and section IDs, including base-*, during the path migration.

This change improves ownership and navigation. Reducing loaded context requires the separate dependency/delivery work.

Acceptance criteria

• Coordinate final ownership with The hourglass has inverted: half the core is an operating manual, not engineering policy #1480 so files move once. If the coherent restructuring warrants an ADR under Stop routine maintenance from generating unnecessary ADRs #1506, share one record across the related work; preserve ADR-005 as history.
• Move the six concern directories out of base/ without changing IDs.
• Update manifest paths, dependency headers, tooling/test assumptions, documentation, and example references; regenerate derived artifacts.
• Account for every tracked old-path reference, distinguishing active references from intentional history.
• Verify the same ordered template IDs resolve for every stack and opt-in root before/after. Keep semantic dependency changes separate.
• Pass existing smoke, synchronization, generated-chain, redundancy, and applicable conformance checks.
• Publish the consumer path mapping and a v3.0 changelog entry, covering vendored paths and reference/hybrid files. Explain how consumers remain on v2 until migrated.

Related work

• The hourglass has inverted: half the core is an operating manual, not engineering policy #1480 — core/process ownership; absorbs duplicate Define the core template boundary separately from always-loaded membership #1505.
• Restructure rule delivery around progressive disclosure (references load nothing on demand) #712 — rule delivery.
• Extract abstract frontend-content layer from frontend-static-site #492 and Decide whether server-side templating belongs in the backend or frontend layer #562 — content and server-side templating boundaries.
• Decide the guide docs' name and casing together, so the files are renamed once #1291 and Evaluate splitting the dev journal into one file per session #715 — guide naming and journal layout.

[braboj]

Raising to P1: this path move is step 2 of the v3.0 split. It moves paths only, IDs stay unchanged, and every root must resolve the same ordered ID list before and after.