Retire the Squadron product docs, glossary and register entries #465

Part of #412. Milestone: Squadron fold. Lands last. Each slice also updates the docs it touches.

Outcome

The product docs describe projects as the unit, and no retired Squadron text is left behind.

Scope

• Retire docs/j5/product/features/squadron.md and features/shared-squadrons.md.
• Glossary (docs/j5/product/glossary.md): remove Squadron, Squadron home, Registrar, Shared Squadron, Squadron scope, and the epic → Squadron note. Rewrite ledger, Exchange and Fleet page in terms of projects.
• Register (docs/j5/product/upstream.md): retire D8 (if the archive slice hasn't already), D9, D10 and D11. Rewrite D12 and D22 where they mention Squadrons.
• Also update crews.md, fleet-page.md, the overview, cross-device.md, docs/j5/plans/dogfood-v0.md (DV5 cohorts) and the user docs.
• Record the reversal: J5 now accepts upstream's one-project-per-folder model, and "several named efforts over one repository" is dropped (Retire Squadrons and fold their J5 behavior into projects (decided) #412, "Lost").

Done when

• git grep -i squadron -- docs returns only historical decision logs.

Closes

#356 (multi-folder Squadrons), and the rest of #412's close list not closed by earlier slices: #38, #39, #48, #128, #139, #154, #295. Most are already obsolete once the fold stack lands. Check each one and close it with a pointer to the PR that made it moot. Reframe #40 (mobile parity) to drop Squadrons.