Add rcdn -D cleanup for dangling managed symlinks

[alexg0]

Summary

Add rcdn -D cleanup for dangling managed symlinks.
The cleanup respects selected dotfiles roots and FILE arguments, exclusions and inclusion overrides, and host/tag selection.
It preserves hooks and ambiguous source paths rather than treating them as removable managed links.

The earlier follow-up fixed multiple selected FILE arguments and dotfiles roots, preserved spaces/backslashes in link names, and replaced invalid removal assertions (-e is already false for dangling links) with -h checks.

Selection fix (2026-09-09)

The previous -D pass checked root membership and positional filenames but skipped exclusion and host/tag scope.
The expanded regression reproduced deletion of links that should have been preserved before this fix.

• Reuse the existing exclusion helpers by moving them unchanged from lsrc into the shared library; apply them to missing source paths and excluded ancestors.
• Respect CLI/config selection and parse the -I argument correctly.
• Keep exclusion globs literal while matching, even when the caller's working directory contains matching live files.
• Preserve source paths containing repeated separators or dot segments, preventing ambiguous classification and an ancestor-walk loop.
• Match explicit nested FILE exclusion spelling consistently with lsrc.

Verification

• Full make check: all 48 integration cases pass.
• Expanded dangling-link regressions cover exclusions, inclusion overrides, tags, hosts, hooks, caller-directory glob expansion, and ambiguous paths.
• Shell syntax checks and git diff --check pass.
• Independent correctness and ponytail reviews completed on the final patch; no outstanding findings or justified complexity cuts.

Current upstream main at f1e5b2d was included in the earlier follow-up; published history is preserved.
These are local test results, not a claim of hosted CI or deployment verification.