Switch the native-backend marker to a bolt glyph in the sidebar badge row

[h0x91b]

Design follow-up to #1233, both changes requested by the user after seeing the shipped marker.

• Glyph: the marker is now a bolt in a rounded frame ("launched straight, without tmux in between") instead of the terminal frame. Same drawing contract as the rest of the icon vocabulary: viewBox 24, currentColor stroke 1.9, round caps.
• Placement: in the Active Tasks sidebar the marker moved out of the title line and into the top badge row, between the priority badge and the agent badge — it qualifies the task, not the agent that runs it. Kanban card and Task View breadcrumb keep their existing position.

Semantics are unchanged: explicit native only, via the shared terminal-backend codec; tmux and legacy records stay unmarked; the marker is non-interactive and still says nothing about liveness. The sidebar row's aria-label keeps carrying the backend label, since that name overrides its descendants.

docs/ux (Bible §5.6 + ux-architecture.yaml) updated for the new glyph and placement.