Docs: after the 6.0.0 release

[CakeRepository]

Brings the docs in line with what shipped in 6.0.0 and how the release actually ran.

• RELEASE_NOTES.md: adds 6.0.0 (the WPF app on .NET 10: wake-word and voice-command phrase variants, the new overlay visualizer and tray icon, --show/--workspace, the portable core underneath) and the 5.1.0 entry that was missing.
• AGENTS.md:
  • .NET 10 instead of .NET 8;
  • the src/ and tests/ projects in the layout table;
  • online-only installers (the offline project isn't built, and there's no finish-page launch);
  • a Releases section: merging into main publishes nothing, a vX.Y.Z tag does, and releases need the owner's go-ahead.
• installer/README.md and README.md:
  • the installers package only the WPF app;
  • the release steps as they run today;
  • a set-but-failing signing secret stops the run before anything is published;
  • the WinGet and Chocolatey steps never fail the release, so check their logs;
  • how to fix the WinGet fork that couldn't sync. That's what 6.0.0 needed: the tag run's WinGet step warned, the fork was synced, and the step was re-run with submit_winget_only, which opened Add FlowDevs.PrimeDictate version 6.0.0 microsoft/winget-pkgs#443985.
• docs/architecture/stage-7-dictation-status.md: merged into main, and what's still missing before the new app can ship (Whisper.net, an updater, its own installer).

Docs only; no code changes.

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