v1.0.20 — @focus global-workspace tier

@Focus — the global-workspace tier

Adds @focus / the perseus_focus MCP tool: a small, capacity-bounded (default 32), salience-ranked working set that Perseus broadcasts into the rendered context — the shared "what I'm working on now" set for an agent and its subagents. The external, orchestration-layer analog of a global workspace.

• add/pin/unpin/drop/touch/clear; salience = weight × frequency × recency-decay; lowest-salience non-pinned items evicted on overflow, pinned protected.
• Opt-in Vault graph-centrality salience (focus.centrality.enabled): boost by degree centrality in the Perseus Vault memory graph; off by default, graceful fallback.
• Distinct from unbounded recall (@mimir/@memory).

Shipped via #689; released via #690.