ci: tighten action-smoke assertion to exact no-bump values

[lesnik512]

Summary

Per the final reviewer of #10: replace test -n (non-empty) with exact-equality assertions so regressions in action.yml's status normalization or bump enum drift surface loudly instead of passing silently.

Before:

test -n "${{ steps.semvertag.outputs.status }}"
test -n "${{ steps.semvertag.outputs.bump }}"

After:

test "${{ steps.semvertag.outputs.status }}" = "no-bump"
test "${{ steps.semvertag.outputs.bump }}"   = "none"
test -z "${{ steps.semvertag.outputs.tag }}"

Test plan

• CI green: the new assertion passes on this PR's no-bump commit (any feat/* or bugfix/* PR tip is a non-merge commit, branch-prefix returns no-bump, all three assertions hold).