perf: skip the per-skill realpath in skill discovery when no workspace snapshot exists #1382

Found during the v0.12.4 release review. Deferred because it is a small, correctness-preserving optimisation rather than a bug.

withoutForeignSnapshots in packages/opencode/src/skill/index.ts (added in #1377) calls fs.realpath on every skill file discovery finds, on every discovery pass, for every user — including users who have never enabled workspaces and have no .altimate-code/skill/_workspace directory anywhere. For them the check can never withhold anything, so the cost buys nothing.

Suggested fix: skip the realpath pass when no match's path contains a _workspace snapshot segment and no snapshot directory exists under the scanned roots, keeping the current fail-closed behaviour whenever a snapshot is present. The symlink cases covered by the "workspace snapshot attribution" tests in test/skill/skill.test.ts must keep passing.