Proposal: support inline template content in schema artifact definitions

[stanleykao72]

Motivation

Schema artifact definitions currently require template to be a filename resolved under schemas/<name>/templates/. For small schemas (a one-artifact "memo" workflow, quick project-local schemas), forcing a separate template file adds friction — the schema author has to maintain a second file for what is often a three-line skeleton.

Proposal

Let template accept either form, distinguished by shape:

artifacts:
  - id: memo
    template: memo.md          # existing: filename under templates/
  - id: memo
    template: |                # new: inline content, used verbatim
      ## Memo

      <!-- What changed, why, which files affected -->

Detection: a single-line, path-shaped string with an extension is treated as a file reference; anything multi-line or not path-shaped is inline content. Fully backward-compatible — every existing schema keeps resolving as before, and a missing referenced file still raises TemplateLoadError.

Implementation

A ready implementation (+50 lines including tests, no behavior change for existing schemas) is on this branch:

https://github.com/stanleykao72/OpenSpec/tree/feat/inline-template-content

Happy to open a PR if there's interest.