Establish the docs/ structure as the canonical location for project
documentation that bridges business need and technical reality.

Adds:
- docs/README.md: tech spec template covering overview, goals/non-goals,
  current state, solution, constraints, open questions, and glossary.
- docs/adrs/: architectural decision records using an extended Michael
  Nygard format (Title, Status, Context, Decision, Consequences,
  Alternatives Considered). Includes README documenting when to write
  an ADR, the lifecycle, and numbering conventions.
- docs/runbooks/: operational runbook template optimized for
  high-stress reading. Includes README codifying when to write one
  and the maintenance discipline (review-on-use, quarterly audit).

Each subdirectory contains both a curated README index and a template
file. Templates use the same blockquote convention as the rest of the
repository (delete-after-instantiation).

This is the foundation for follow-up work: Makefile generation
targets (make adr / make runbook), CI-enforced docs hygiene
(adr-lint with pre-commit and GitHub Actions integration), staleness
detection, and broader documentation automation. Those land in
subsequent commits to keep this change reviewable in isolation.

Add .tool-versions as the source of truth for runtime versions, with
Makefile bootstrap detection and CONTRIBUTING.md updates to recommend
mise to new contributors.

Changes:
- .tool-versions: template file with conventions documented inline.
  All entries commented out; projects derived from this template
  uncomment the lines they need.
- Makefile: HAS_TOOLVERSIONS detection, bootstrap step that runs
  mise install (or asdf as fallback) when .tool-versions is present,
  graceful degradation with instructional message when neither
  tool is installed.
- CONTRIBUTING.md: prerequisites updated to recommend mise as the
  primary version manager. Existing language-specific manifests
  (package.json engines, pyproject.toml, etc.) are kept but framed
  as derivative of .tool-versions.

Eliminates the duplication of version specs across CONTRIBUTING.md,
CI workflows, Dockerfiles, and language manifests. Bumping a tool
version becomes a single-file change.