Add markdown task import to task creation dialog

[masonjames]

Summary

Add a markdown import flow to the New Task dialog so users can bootstrap backlog cards from PRD/strategy files in the current workspace.

This touches many files so I understand if it's overwhelming as a first PR. My workflow always starts with a PRD. Pulling that directly into the kanban board is the first thing I wanted when testing the app.

Since this would be my top feature request, rather than ask for it I decided to take a crack at it with codex's help.

Included

• workspace markdown file search + safe read support for .md, .markdown, and .mdx
• markdown task parsing into editable task rows
• create imported tasks in source order
• web-ui Vitest storage hardening for environments with incomplete window.localStorage shims
• tests for runtime validation, parsing, ordering, dialog behavior, and storage fallback

Acceptance criteria

• A user can search for and select a workspace markdown file from the New Task dialog
• Imported markdown tasks are extracted into editable task rows
• Created backlog cards preserve source document order
• Backing out of imported multi-task mode restores the original single prompt
• Web UI tests remain stable when window.localStorage is incomplete in Vitest/jsdom
• CI passes

Verification

Ran locally:

• npm run build
• npm run check
• npm --prefix web-ui run test