feat: add active-only Tasks filter

[christoph-hess-itemis]

Summary

• add an enabled-by-default Active only switch to the built-in Tasks topology
• hide completed/inactive subagents while retaining active ancestor branches and the currently selected branch
• persist the preference as hideCompletedSubagents and expose it in Side Card settings
• add accessible empty-state behavior, complete locale coverage, docs, and regression tests

Architecture

This is implemented natively in Better Sidebar rather than as a separate consumer plugin because the public betterSidebar service can register tabs/viewers but cannot extend the contents of the built-in Tasks tab.

Validation

• pnpm typecheck
• pnpm build
• bash scripts/check-consumer-types.sh
• focused Vitest: 86 tests passed across 7 suites
• git diff --check

Environment-only failures

• Full pnpm test reaches 27 pre-existing/environmental failures: 26 PTY cases fail with posix_spawnp failed; one unrelated git pagination smoke assertion also fails.
• pnpm test:mount reaches installation but the local DSH runtime has package version skew: @deepseek-ai/dsh-settings does not export settingsNamespace expected by the installed runtime.