feat: add cursor-drained event subscriptions

[umputun]

implements the event subscription accepted in Discussion #211 with a bounded in-memory ring and ordinary one-shot events.read cursor reads. agtermctl events short-polls the API and prints human or NDJSON output without changing the control server's connection lifecycle.

v1 emits status, notify, session.created, session.closed, and tree.changed. The structural event is a per-window coalesced invalidation; consumers re-read tree --json for detail. Cursors are scoped to an app run and fail explicitly on restart, expiry, or invalid positions.

a minimal client that jumps to a session when it reports blocked:

agtermctl events --json --kind status |
  jq -r --unbuffered 'select(.payload.status == "blocked") | .session' |
  while IFS= read -r session; do
    agtermctl session select --target "$session"
  done

tested with:

• cd agtermCore && swift test (1,727 tests)
• focused EventSubscriptionUITests
• make build
• make lint

discussion: #211

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