Fix mount filter to use target paths instead of source prefixes

[dguido]

Problem

extract_mounts_to_file in install.sh filters default mounts by source prefix:

(startswith("source=claude-code-bashhistory-") | not) and
(startswith("source=claude-code-config-") | not) and
...

This is hardcoded to the claude-code volume prefix. If someone forks this and changes the volume names, or if the naming convention changes, the filter silently stops working and default mounts get preserved/duplicated on template reinstall.

Proposed fix

Filter by target path instead, which is constant regardless of volume naming:

(contains("target=/commandhistory,") | not) and
(contains("target=/home/vscode/.claude,") | not) and
(contains("target=/home/vscode/.config/gh,") | not) and
(contains("target=/home/vscode/.gitconfig,") | not) and
(contains("target=/workspace/.devcontainer,") | not)

Also fix the temp file leak — add rm -f "$temp_file" in the else branch when no custom mounts are found.

Already done in trailofbits/skills devcontainer-setup plugin.