Collect existing bugreports before generating a new report

[besendorf]

AndroidQF currently generates a new bugreport but does not copy reports already retained on the device. Those older reports may contain useful historical evidence, and generating another report can trigger Android's retention cleanup.

This change collects existing files accessible through /bugreports/ before generating the fresh report. Existing reports and companion files retain their relative names under bugreports/ in the acquisition archive; the newly generated report remains at bugreport.zip.

Collection follows the top-level directory alias but does not follow symlinks inside it. Files are staged before archiving so failed transfers do not leave truncated entries. Missing or empty directories are accepted; listing or transfer failures mark the collection partial while still allowing fresh bugreport generation. Existing source files are not deleted.

The scope is intentionally limited to /bugreports/. The issue discussion confirms that this aliases the longer shell-app directory on a tested Samsung device. Exported copies in shared storage and vendor-specific locations are not searched. The README documents that Android may already have deleted older reports, so an empty directory does not establish that no reports were generated.

Validation: go test -race -tags unbundle ./... passed. Tests cover collection before generation, the top-level directory symlink, skipping internal symlinks, filenames containing spaces/newlines/quotes, absent and empty directories, unsafe paths, and listing, transfer, and generation failures. No physical-device test was performed.

Fixes #50