chore(specgit): pin specgit CLI version in the acceptance workflow

[LeXwDeX]

Why

npm install -g specgit in specgit-accept.yml installs whatever npm considers latest at run time. The SpecGit CLI is under active development with multiple releases per day; an unnoticed semanic change would flip CI acceptance verdicts repo-wide with no corresponding change in this repo.

What

• Pin to specgit@^0.5.0 (caret for patch/minor fixes, floor for reproducibility).

Acceptance

• acceptance workflow installs a pinned-at-run version and passes on the delivery PR

[LeXwDeX]

Delivered via PR #367 (merged to dev); acceptance now installs specgit@^0.5.0 (caret floor tracks latest per maintainer direction). specgit finish exit 0.