docs: prepare v0.5.79 release notes

[yannickmonney]

Prepare v0.5.79 forward-release notes for API-key expiry controls, chat attachment access, erasure receipts, automation parsing, private deployment-control routing, compact task occupancy reads, sandbox overhead and cleanup, knowledge vector widths, safe CLI Git diagnostics, and the managed automation cutover barrier. Explain the stopped v0.5.78 publication and the required migration, compatibility and rollout steps.

This remains a draft. Landing requires the final #4565 composition, including #4569 and #4573, and the reviewed #4579 managed cutover to reach main, followed by reconciliation against that exact source. The child PR links remain provenance for their changes. These notes do not establish a completed release or deployment. Candidate selection, tagging and rollout still require their maintained gates and a safe protocol cutover.

Validation: maintained release-note validator and diff check passed. Markdown is excluded by repository formatting configuration; the stdin formatter returned the final text unchanged, and normal commit hooks passed. Independent factual review and root review accepted the final prose. The complete local Markdown page was inspected through T3 preview at 1280px and 390px, with correct section order, links and no narrow horizontal overflow. This is documentation verification, not deployed-product verification. Exact-head PR CI and final source reconciliation remain pending.

[yannickmonney]

The reviewed v0.5.79 notes were incorporated and updated in #4565, merged as 0ee5fd9656dc12f08976155d956ee76fe39f4ea3. Closing this superseded draft. Release publication and deployment remain separate from this source merge.