0.20.0 — Watson never works a blocked item

Dr. Watson now refuses to begin or resume work on a blocked issue.

• 🚫 Blocker gate before any work. A new step 2.5 in Watson's Index-mode pipeline runs right after the state fetch and before resume detection: if the item has an open blocker, Watson leaves it exactly where it is — Ready stays Ready, In Progress stays In Progress (frozen, never demoted or abandoned), releases the lock, and exits reporting the blocking issue(s).
• 🔒 Fail-closed. Watson asks get_item(blockers: true) for live blocker state; if the check can't run (has_open_blockers: null), it's treated as blocked. Only an explicit false proceeds — "can't confirm it's safe" means "don't touch it."
• 🧩 Defense in depth. list_development_items already filters blocked items out of the autonomous queue (and resumes them In-Progress-first the moment the blocker closes); this gate is the safety net for direct dispatch by ID, where selection is bypassed.
• 🗂️ Requires The Index get_item blockers param (mike-bronner/the-index#20) — deploy that first.

PR #32.