v0.12.0

Release 0.12.0

Highlights:
- MCP: background file watcher keeps the entity graph live (reflects
  uncommitted edits; no per-call re-walk/re-stat).
- sem context prints the target entity's full body in the terminal;
  sem context / sem impact (CLI + MCP) accept Class.method addressing.
- Spinner now shows rotating command tips, and a once-a-day cloud nudge
  after a slow local build for logged-out users. sem diff shows the
  spinner during its compute.
- Resolver: super::/crate::/self:: qualified calls resolve to the real
  entity. Graph build perf: faster hash collections, cached reference
  lookups, lower peak memory.