The GitLab CI component has no `dry-run` input

[lesnik512]

The two distribution wrappers are not symmetric. action.yml exposes a dry-run input and threads
it to semvertag tag --dry-run; templates/semvertag.yml exposes only strategy and always runs
tag for real. A GitLab user has no supported way to preview the bump, and the safe pattern this
repo uses on itself — dogfood in dry-run so CI can exercise the wrapper without minting tags — is
unavailable to them.

The change mirrors what action.yml already does: add a dry-run input to the spec.inputs block,
map it onto the CLI flag in script:, and extend tests/_descriptor_gate.py (which currently
asserts set(inputs) == {"strategy"}, so it will fail until updated) plus its negative fixtures in
tests/test_ci_descriptor_gate.py.

While in there: the component pins uvx 'semvertag>=0.1,<1', a floor from before --dry-run existed
(0.5.0). action.yml pins >=0.5.0,<1. The GitLab floor has to move to at least >=0.5.0,<1 for a
dry-run input to be safe.

Revisit trigger: none — actionable now. It is unscheduled because the CLI change shipped in
0.5.0 and only the GitHub half of the follow-up was ever specced.

Origin: recorded only in architecture/cli.md ("the GitLab component has no dry-run path today"),
deleted by the planning/ migration.