Workbook v0.5.1

Five stories from adversarially reviewed pull requests: the web board learns
to edit assignments and accept pasted images, and three defects found in
earlier reviews are closed.

Added

• Assignments can be added and withdrawn from the web board, through the same
  claim semantics the CLI uses (#121).
• An image in the clipboard pastes straight onto the attachment controls, with
  a generated filename and the same ceilings as drag-and-drop (#124).

Fixed

• With no sync watcher answering, the board's Publishing switch shows a
  stalled state naming why, instead of silently flipping server configuration
  it cannot apply (#123).
• Terminal output neutralizes Unicode bidi-control format characters, so a
  hostile title or comment can no longer visually reorder a rendered line
  (RTLO spoofing); benign format characters like emoji ZWJ pass through
  (#122).
• workbook validate now refuses a history containing duplicate operation
  ULIDs as corrupt-data, and the projection reports the offending task instead
  of crashing with a raw SQLite constraint error (#120).