Fix Dired DWIM context fallback

[rpoisel]

Dired's current-line fallback is useful for explicit file sending, but it should not count as obvious context when starting or reusing a shell.

Only use region-selected or explicitly marked Dired files for automatic file context, and skip current-line context in Dired so raw listing rows are not inserted into prompts.

Note: I am not really good at writing elisp code. This commit has been created with the assistance of an AI agent (OpenAI Codex). I do understand the code, but there might be better ways to improve the situation. It works for me - I am typically starting agent-shell sessions from dired buffers without selecting entries (files, directories, etc.).

Checklist

• I agree to communicate (PR description and comments) with the author myself (not AI-generated).
• I've reviewed all code in PR myself and will vouch for its quality.
• I've read and followed the Contributing guidelines.
• I've filed a feature request/discussion for a new feature the observed behavior: Dired DWIM context can insert prompt garbage into new agent sessions #788
• I'm making visual changes, so I'm including screenshots so you can view and discuss.
• I've added tests where applicable.
• [] I've updated documentation where necessary.
• I've run M-x checkdoc and M-x byte-compile-file.