feat(pr): add --attach for GitHub pull requests (#43)

* feat(pr): add --attach for GitHub pull requests

GitHub CLI 2.99.0 can upload images and videos on `gh pr create --attach`.
`pq pr` and `pq submit` now accept the same flag when they create a new PR.

Parse `path[#alt]` into `PRAttachment` at the CLI. Validate that each path is
a file, reject duplicates, and cap the list at 50. Forward the values through
`GitHubPort.create_pr`.

`verify-panqake` writes a 1x1 PNG for attach recipes and pins GitHub CLI
2.99.0 or newer onto the fixture PATH.

Co-authored-by: SarthakJariwala <SarthakJariwala@users.noreply.github.com>

* chore(pr): drop restating helper docstrings

The helpers already name parse, validate, and pull-request URL extraction.

Co-authored-by: SarthakJariwala <SarthakJariwala@users.noreply.github.com>

* fix(pr): stop treating failed gh create as success

A nonzero `gh pr create` is success only when gh printed a pull request
URL. That matches the partial `--attach` upload contract. An existing PR
for the head is not treated as success.

PRCreationError now includes gh stderr. The 2.99.0 hint fires only when
stderr reports an unknown `--attach` flag. Relative `--attach ./file`
spellings are kept so gh can rewrite matching body paths.

Co-authored-by: SarthakJariwala <SarthakJariwala@users.noreply.github.com>

* fix(pr): seed PR body from --attach when description is empty

--defaults (and other empty descriptions) previously passed --body "" to
gh pr create. GitHub CLI then left attachments unreferenced in the body,
so the image did not show on the PR. Fill that empty body with ![alt](path)
so gh can rewrite the upload URL in place.

Co-authored-by: SarthakJariwala <SarthakJariwala@users.noreply.github.com>

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Co-authored-by: Cursor Agent <cursoragent@cursor.com>
Co-authored-by: SarthakJariwala <SarthakJariwala@users.noreply.github.com>

Author: SarthakJariwala