[DIST-05] Publish the Debian and Ubuntu APT repository

[sodejm]

Outcome

Publish supported amd64 and arm64 Debian packages through a signed APT repository for the desktop client and advanced headless server entrypoints.

Scope

• Produce architecture-specific Debian packages from the canonical release artifacts.
• Install the repository key through a dedicated keyring and use a signed-by source definition; never use deprecated global apt-key trust.
• Separate desktop-client and headless remote-server packaging when that produces safer dependencies and clearer operator intent.
• Do not bundle or silently install a container daemon; declare supported prerequisites and provide actionable checks.
• Reuse [DIST-01] Define the cross-platform package-manager bootstrap contract #358 for Local Desktop, Connect to Remote Server, and advanced Host Remote Server behavior.
• Make maintainer scripts minimal, idempotent, non-interactive-safe, and data-preserving.

Security and privacy requirements

• Serve repository metadata and artifacts over TLS and verify signed Release/InRelease metadata plus package hashes.
• Scope repository trust to the AncestryLLM source; do not add a globally trusted package key.
• Do not open firewall ports, create public listeners, generate weak credentials, or select server mode during package installation.
• Keep secrets out of package configuration, debconf output, process arguments, and logs.
• Normal purge behavior must distinguish application files from encrypted user data and require explicit confirmation before destroying data.

Acceptance criteria

• amd64 and arm64 packages install and upgrade on the supported Debian and Ubuntu matrix.
• APT repository metadata is reproducible, signed for 1.0, and validated in release automation.
• Repository onboarding uses a scoped keyring and signed-by source configuration.
• Desktop and server package boundaries, dependencies, services, upgrade behavior, rollback, and removal are documented and tested.
• No install path produces an unauthenticated listener or silently selects remote-server mode.
• User, advanced-operator, release, privacy, and troubleshooting documentation is complete.

Dependencies

Depends on #132, #355, #356, #358, and the Linux container-runtime validation work.

Reference

Debian guidance for third-party repositories: https://wiki.debian.org/DebianRepository/UseThirdParty