3747-4: abort decider + sentinel (~200 LOC code)

[nikw9944]

Parent issue: #3747

Split from plan review of #3747 (estimated ~200 lines).

PR stack: this issue's branch should target nikw9944/doublezero-3795 (the previous sub-issue's branch).

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• Creates internal/abort/decider.go (central state machine, rolling
  windows, p95 helpers, sentinel writer).
• Wires all collector outputs into the decider (replacing PR 1 stub
  channels).
• Adds startup check that refuses to start when a stale abort exists
  in --working-dir unless --force.
• Adds unit test per trigger (full list in "Files to Change → New (tests)"
  below).
• Updates README with abort-trigger table and operator-contract section.

Each chunk leaves the binary in a working, runnable state with the
previously shipped behavior intact.