fix(skills/understand): canonicalize isCli paths so symlinked SKILL_DIR runs main()

import.meta.url resolves through symlinks but pathToFileURL(process.argv[1])
preserves them, so extract-structure.mjs silently exited 0 without writing
output when invoked via the plugin's symlinked install path — the documented
Claude Code / Copilot CLI layout. Compare both sides via realpathSync and add
a post-write existence assertion plus caller-side guidance in the agent.

Closes #162