docs: document spec persistence models

[wondr-wclabs]

Description

Adds a new concept page that documents three spec-persistence models discussed in #2831:

• flow-back spec
• flow-forward spec
• living spec

The page explains that Spec Kit does not enforce or prefer any one model, then links the new page from the SDD concept page and docs table of contents.

Closes #2832

Testing

• npx --yes markdownlint-cli2 docs/concepts/spec-persistence.md docs/concepts/sdd.md
• git diff --check
• ruby -e 'require "yaml"; YAML.load_file("docs/toc.yml"); puts "toc ok"'
• Tested locally with uv run specify --help (not applicable: docs-only change)
• Ran existing tests with uv sync && uv run pytest (not run: docs-only change)
• Tested with a sample project (not applicable: docs-only change)

DocFX was not run locally because dotnet/docfx are not installed in my environment.

AI Disclosure

• I did not use AI assistance for this contribution
• I did use AI assistance (describe below)

This PR was written with AI agent assistance in Codex. I reviewed the issue/discussion context, followed the contribution guidance, made the docs changes, and ran the validation listed above.