Describe the bug
sync_integration() in skill_integrator.py hard-codes skill directory
prefixes (e.g. ".opencode/skills/") when scanning managed files for
removal. User-scope deployments can produce roots with nested path separators
(e.g. ".config/opencode/skills/"), which do not match the hard-coded
prefixes. This means user-scope skill directories are not cleaned up during
uninstall/sync.

To Reproduce

1. apm install -g --target opencode (deploys to ~/.config/opencode/skills/)
2. apm uninstall -g <package> (sync_integration tries to remove orphaned skills)
3. Hard-coded prefix ".opencode/skills/" does not match ".config/opencode/skills/"
4. Orphaned skill directories remain under ~/.config/opencode/skills/

Expected behavior
sync_integration() should derive allowed skill prefixes from KNOWN_TARGETS
using target.effective_root(user_scope=...) (and deploy_root overrides)
instead of hard-coded strings.

Environment (please complete the following information):

• OS: All (code-level bug, not platform-specific)
• Python Version: 3.12
• APM Version: 0.8.9
• Affected file: src/apm_cli/integration/skill_integrator.py
  (sync_integration() method)

Logs
N/A -- no runtime error; orphaned directories silently remain.

Additional context
Pre-existing issue not introduced by #537, but surfaced during Copilot code
review. Affects any target where user_root_dir != root_dir (currently
Copilot and OpenCode).

Suggested approach: build the prefix set dynamically from
KNOWN_TARGETS.values() iterating over each target's effective_root()
for both scopes, similar to how get_integration_prefixes() works in
base_integrator.py.

Related: #537, #530