Speed up the default bioccheck.yml path: conditional installs and drop single-element matrix #34

Two runtime changes deferred from #33 because they alter behaviour rather than docs.

1. Install Package Dependencies installs pkgdown, cyclocomp and covr on every run, even when those features are off. pkgdown in particular pulls in a large dependency tree. Install each only in the step that uses it, or gate the install on the corresponding input / token.
2. set-matrix emits a one-element docker_tag array and check uses strategy.matrix + fromJson only to unwrap it. Since there's only ever one tag, check can read needs.set-matrix.outputs.docker_tag directly and the strategy block goes away. The matrix is a leftover from the upstream multi-tag design (ADR 0001 rejected matrix testing for this workflow).

The canary run after each change is the test.

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