#1977 fix(cli): warn that fleet release does not tear down Cloud sandboxes

Summary

A successful fleet release dispatch can leave a Cloud/provider sandbox running and incurring cost. Warn on stderr after a dispatched acknowledgement while retaining the existing single JSON document on stdout, including --delete-agent and --delete-agent --wait. This change does not request sandbox teardown.

Fixes #1922.

Adds seven regression tests, one RelayFlow bugfix proof case, a patch changelog entry, and tracked Trail records.

Test Plan

• Tests added/updated: all 18 release tests pass, including the seven new warning/output/failure regressions.
• Manual testing completed: ran the production command registration with the default output sinks and a stubbed remote release; separately captured stderr and parsed stdout as exactly one unchanged JSON document.
• npm run typecheck passes with Git's Unix tools on PATH for the existing build scripts.
• Changed source/proof files pass Prettier; ESLint reports no errors (existing complexity/depth/error-cause warnings, plus the release handler's complexity increases from 15 to 16).
• Proof manifest validation and git diff --check pass. The same runner observes bug on exact base c1ffabbc4 and fixed on exact head d4119014e; neither observation is selected from the proof arm.

Validation limits: the complete fleet file has 114 passing tests and one Windows path failure in --checkout keeps cross-repo --cwd inference on the actual checkout while using the explicit project workspace (/workspace-project versus C:\workspace-project). Unchanged base reproduces the same failure with 107 passing tests. The broader Windows suite (npm test --ignore-scripts -- --reporter=json) did not pass: its report records 3,040 passed, 342 failed, and 38 pending tests, and a worker ran out of heap memory. This PR remains draft pending maintainer validation on Linux. Fork PRs also require a maintainer to reproduce the branch and proof case in the upstream repository before the credential-bearing Cloud proof can run, per the case README.

RelayFlow Proof

• Change type: bugfix
• RelayFlow case: 1922-fleet-release-sandbox-warning

Screenshots

Not applicable; CLI warning only.

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