fix: copier should not be a dev dependency

[coretl]

Summary

copier should not be installed as a dev dependency of generated projects — it should be invoked via uvx copier instead. This PR removes it from the template's dev dependency group and updates the docs to recommend uvx copier update.

• template/pyproject.toml.jinja: removed "copier" from the dev dependency-group list.
• docs/how-to/update-template.md: changed the two bare copier update invocations to uvx copier update.
• .github/CONTRIBUTING.md: changed copier update --vcs-ref=<branch_name> to uvx copier update --vcs-ref=<branch_name>.

Deliberately left untouched

• Root pyproject.toml — this template repo's own test suite imports copier (from copier import run_copy), so it legitimately needs copier as a dev dependency here.
• tests/test_example.py — its bare copier update call runs in this repo's own dev environment, not inside a generated project.
• Tutorial docs (docs/tutorials/adopt-existing.md, docs/tutorials/create-new.md) and prose/comment mentions of "copier" — already correct or out of scope.

Testing

• uv run --locked tox -e pre-commit — passed.
• uv run --locked tox -e tests — 5 pre-existing failures unrelated to this change (sandbox network proxy blocks a docs-build version-switcher request to diamondlightsource.github.io, and one test has a version-string mismatch from running off an untagged commit rather than a release tag). No failures caused by this change; test_example_repo_updates's actual file-content diff was clean aside from that unrelated version string.

Closes #343

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Generated by Claude Code

[coretl]

Summary

Removed copier from the generated project's dev dependency group (template/pyproject.toml.jinja) and updated the docs (docs/how-to/update-template.md, .github/CONTRIBUTING.md) to recommend uvx copier update instead of a bare copier update.

CI status

All checks passed:

• lint / run ✅
• test / run ✅
• docs / build ✅
• validate-pr-title ✅
• example / release — skipped (not applicable to this PR)

Locally, tox -e tests had shown some failures caused by this sandbox's network policy blocking a docs-build request and an untagged-commit version-string mismatch; neither reproduced in the real CI run, confirming they were sandbox-only artifacts unrelated to this change.

Deliberately left untouched

• Root pyproject.toml — this template repo's own test suite imports copier (from copier import run_copy), so it legitimately needs it as a dev dependency here.
• tests/test_example.py — its bare copier update call runs in this repo's own dev environment, not inside a generated project.
• Tutorial docs and prose/code-comment mentions of "copier" — already correct or out of scope.

Closes #343

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Generated by Claude Code