docs: tasks page redesign plan + README/guide sync
- docs/plans/2026-09-14-tasks-graph-workflow-teams-design.md: data-source
facts, degradation matrix, tradeoffs, and the implementation-deviation
record (diagnostics banner, mount-e2e desktop-shim pitfall, portal
assertions).
- README feature list/table + external-plugin-guide tab registry row now
describe the workflow-graph page, team task board, and jobs drawer.
- guideDescSubagent reworded in zh/en/ja (third-language retranslations
follow separately).
Menghuan1918
b1bfaf8730