docs: add API versioning strategy guide

[PAMulligan]

Summary

Adds docs/api-development/api-versioning.md, a guide to versioning a Nerva/Hono API and evolving it without breaking consumers. Nerva ships an opinionated default (URL prefix, /api/v1/); this guide explains that default, the alternatives, and how to evolve safely whichever you pick.

What it covers

• Nerva's default — URL prefix, quoting the api.versioning block of pipeline.config.json (strategy: "url-prefix", currentVersion: "v1", deprecationHeaderEnabled: true) and why URL-prefix is the default
• API version vs. release version — the contract (v1) vs. the deployed build (APP_VERSION)
• All four strategies — URL prefix, custom header, content negotiation, query parameter — in a trade-off table, then a working Hono implementation for each (sub-app mounting, basePath, version-resolving middleware, Accept media-type negotiation with Vary)
• Deprecation and Sunset headers — Deprecation (RFC 9745) and Sunset (RFC 8594), gated by deprecationHeaderEnabled, with a Hono middleware
• Breaking vs. non-breaking changes — two reference lists plus the tolerant-reader principle
• Migration checklist — eight ordered steps for shipping a version bump

Wiring

• References the api.versioning section of .claude/pipeline.config.json (and links it)
• Linked from docs/api-development/README.md via a new API Versioning section

Notes

• Docs-only; the affected CI job is the lychee link check. The RFC and Hono URLs were verified to return 200, all internal link targets exist, and inline-code backticks are balanced.
• The minimal example apps mount routes at the root; the guide documents the intended /api/v1 default per the config and conventions without claiming those examples use it.

Closes #24

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