feat(kanban): surface task outcomes and blocked context

[Lypidri]

Summary

Reopens the Kanban dashboard improvements from #25655 as a fresh branch against current main, without replaying the stale old bundle wholesale.

This updates the selected-task drawer/API so reviewers can see the useful task outcome information first:

• Adds a summary-tree payload for a selected task and descendants.
• Surfaces result/handoff text, latest run summary, run metadata, subtask status/progress, and important comments.
• Adds structured blocked-task context on board/detail/summary-tree payloads: reason, missing info, latest relevant comment, and a comment prompt.
• Adds artifact availability metadata so unavailable/missing/scratch paths render as explicit empty states instead of misleading open/copy actions.
• Keeps local artifact opening restricted to backend-derived, user-actionable artifacts; comment-derived paths are context-only and not openable.
• Updates the current Kanban dashboard bundle carefully rather than reverting it to the old PR's stale state.

Notes on the previous PR feedback

The old PR was closed because its plugins/kanban/dashboard/dist/index.js was stale against current main. This branch starts from current origin/main and ports the behavior onto the current Kanban dashboard code, preserving newer scheduled/review/workflow/orchestration changes.

I looked for a maintained Kanban source file under web/src/plugins/kanban; this bundled dashboard plugin currently ships as a plain non-minified IIFE in plugins/kanban/dashboard/dist, so the frontend changes are applied there while keeping the diff focused.

Test Plan

• git diff --check -- plugins/kanban/dashboard/plugin_api.py plugins/kanban/dashboard/dist/index.js plugins/kanban/dashboard/dist/style.css tests/plugins/test_kanban_dashboard_plugin.py
• node --check plugins/kanban/dashboard/dist/index.js
• $HOME/.hermes/hermes-agent/venv/bin/python -m pytest tests/plugins/test_kanban_dashboard_plugin.py -q -o 'addopts=' — 102 passed
• $HOME/.hermes/hermes-agent/venv/bin/python -m pytest tests/hermes_cli/test_kanban*.py tests/tools/test_kanban_tools.py -q -o 'addopts=' — 620 passed, 1 skipped
• npm --prefix web ci
• npm --prefix web run build

Additional check:

• pytest tests/plugins -q -o 'addopts=' currently reports 751 passed / 1 failed. The failure is unrelated to this diff: tests/plugins/web/test_web_search_provider_plugins.py::TestBundledPluginsRegister::test_all_seven_plugins_present_in_registry expects seven web search providers, but current plugin discovery includes xai as an eighth provider.

[SatoDri]

Superseded by #29247, opened from the SatoDri fork/account.

[Lypidri]

Superseded by #29247, opened from the SatoDri fork/account.

[alt-glitch]

Resubmission of closed #25655 rebased onto current main. Same feature scope: task outcome summaries, blocked context, and artifact availability metadata for the Kanban dashboard.