Fix pelican landing page links for print/single-file targets

[Copilot]

When a target with deploy-dir produces a single output file (PDF, EPUB, etc.), the pelican-generated landing page linked to the deploy directory rather than the actual file.

Changes

• pretext/project/__init__.py: Modified deploy_path() to scan output_dir_abspath() for the actual output file when output_filename is not explicitly set and the format is a known single-file type. Returns deploy_dir / filename if exactly one match is found; falls back to directory path otherwise.

  • Added module-level _SINGLE_FILE_FORMAT_EXTENSIONS constant mapping Format.PDF → .pdf, Format.EPUB / Format.KINDLE → .epub, Format.BRAILLE → .brl

• tests/test_project.py: Added test_deploy_path covering PDF/EPUB auto-detection, explicit output_filename precedence, HTML directory-output unchanged behavior, and fallback when output dir doesn't exist yet.

Before: a PDF print target with deploy-dir="print" would generate <a href="print">Print</a>
After: it generates <a href="print/main.pdf">Print</a>

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• runestone.academy
  • Triggering command: /usr/bin/python python scripts/fetch_core.py (dns block)
  • Triggering command: /usr/bin/python python -m pytest tests/test_project.py::test_deploy tests/test_project.py::test_defaults tests/test_project.py::test_manifest_elaborate tests/test_project.py::test_stage -v (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

Original prompt

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This section details on the original issue you should resolve

<issue_title>Print targets should get better links in pelican generated site</issue_title>
<issue_description>Currently the @deploy-dir getting set in the manifest tells pretext deploy to build a static landing page for multiple projects. If you use this for a print target, the landing page will create a link to that directory. It would be much better to create a link to the actual pdf.</issue_description>

Comments on the Issue (you are @copilot in this section)

@oscarlevin It seems like if the user specifies a `output-filename` then the link points to the correct thing. I suspect this is just an oversight in the pelican generation, using `none` instead of the default, perhaps.

• Fixes Print targets should get better links in pelican generated site #1085

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