docs(issues): publish SI-4 torrent cleanup specification

[josecelano]

Summary

Publishes the approved specification for #2169, SI-4 in the #1488 shutdown roadmap.

• Replaces the outdated pre-spawned JoinHandle proposal with an unspawned, token-aware cleanup runner directly owned by JobManager::spawn.
• Defines explicit normal outcomes for cancellation and weak-manager expiry.
• Requires deterministic lifecycle and named-supervisor-outcome tests.
• Adds direct-binary SIGTERM verification and a functional inactive-peer cleanup scenario through the REST API.
• Requires a separately committed reusable manual cleanup verification skill after the manual scenario establishes the exact procedure.

Related to #2169

Validation

• linter all
• git diff --check
• Mandatory pre-commit checks
• Pre-push checks: nightly format and workspace checks, documentation build, and full test suite

[Copilot]

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Pull request overview

Publishes the approved SI-4 specification for migrating torrent cleanup to CancellationToken, moving it from a draft into the EPIC roadmap as an open, tracked issue (#2169).

Changes:

• Adds a new open issue spec for SI-4 with design, plan, and verification requirements.
• Updates the EPIC #1488 roadmap to point SI-4 to the new open issue (#2169).
• Removes the outdated SI-4 draft specification.

Reviewed changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated 3 comments.

File	Description
docs/issues/open/2169-1488-si-4-migrate-torrent-cleanup/ISSUE.md	New approved SI-4 spec detailing design, ownership invariants, acceptance criteria, and verification.
docs/issues/open/1488-overhaul-tracker-shutdown/ISSUE.md	Updates EPIC roadmap entry to reference the new open SI-4 issue instead of the draft.
docs/issues/drafts/1488-si-4-migrate-torrent-cleanup/ISSUE.md	Removes the superseded draft SI-4 spec.

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[josecelano]

ACK 1cacd68