feat: exit 0 on --filter no-match, add --fail-if-no-match

[kazupon]

Summary

• Match pnpm: vp run --filter <expr> now exits 0 with a warning when the expression matches no packages, instead of failing with Task "X" not found.
• This also covers --filter {.}^... on a leaf package: the seed matches but the traversal collapses to zero, a legitimate no-op rather than a typo.
• Add --fail-if-no-match for callers (CI scripts) that want the previous strict behaviour. In strict mode, any unmatched --filter aborts the run, even when other filters did match.

Implementation notes

• unmatched_selectors now tracks filters whose expanded set is empty (was: whose core selector matched nothing), so traversal collapses are reported too.
• TaskQueryResult exposes selected_package_count and plan_query returns a PlanResult { graph, no_packages_matched } so the CLI can distinguish "no packages matched filter" (succeed) from "packages matched but task missing" (still errors as NoTasksMatched).
• Nested vp run --filter X build inside a task script gets the same default-success treatment.

Closes #380