Fix multiple-installs false positive for asdf shims

[gtsiolis]

Motivation

lstk warns about "multiple installs" whenever an asdf-managed npm install is also reachable through its asdf shim on PATH, even though both paths point at the same install — a false positive that fires on every run for asdf users.

Changes

• Resolve asdf shim scripts back to their real install path (internal/update/installs_unix.go) by parsing the # asdf-plugin: <name> <version> comment asdf writes into each shim, before dedup runs in FindInstalls (internal/update/installs.go)
• Add a no-op executableAlias on Windows (installs_windows.go), where asdf shims don't apply
• Add unit test TestFindInstallsDeduplicatesAsdfShimAlias and integration test TestUpdateCheckDoesNotWarnOnAsdfShimAlias

Review

Human review advised — found and fixed today with no prior Slack/Linear discussion before this PR, even though it's small, self-contained, and fully tested (per the review-pr skill's scope checklist: undiscussed work defaults to review regardless of size).

Tests

New unit + integration tests reproduce the false positive and confirm it's resolved. Also verified directly against a real asdf + npm install on this machine: today's code reports 2 installs for this PATH combination, this branch reports 1, correctly classified as npm.

Closes DEVX-1015

Co-Authored-By: Claude noreply@anthropic.com

[gtsiolis]

Cc @skyrpex because #398.

[skyrpex]

@gtsiolis thanks, didn't know about this. I thought we were fine deduping symlinks 😄