fix(context): sync to the estate canonical context-sync.sh, drop Cursor/Aider

Same drift as gitea-mcp: this repo's copy of context-sync.sh was two days
stale against dma-cli's canonical (Mathias's Cursor/Aider decision,
2026-09-01) and CLAUDE.md itself was stale enough to need 97 lines of
correction on regeneration -- this repo's context:sync was apparently never
wired into `task check` at all (no drift assertion existed in its Taskfile),
so nothing had been catching it.

Found while building the estate-wide context-sync drift gate (ADR-0019).

- script synced to canonical hash
- .cursorrules, .aider.conventions.md, .aider.conf.yml deleted
- stale context:sync:cursor Taskfile shortcut removed (would now hit the
  canonical script's own `exit 1` on an unknown adapter name)
- CLAUDE.md, AGENTS.md, .context/system-prompt.txt regenerated against canonical

task check -> exit 0.

Taskfile.yml

@@ -80,5 +80,3 @@ tasks:
     cmds: [bash scripts/context-sync.sh claude]
   context:sync:agents:
     cmds: [bash scripts/context-sync.sh agents]
-  context:sync:cursor:
-    cmds: [bash scripts/context-sync.sh cursor]