Rename the cross-platform tentacle flavor: LinuxTentacle to Tentacle

Summary

A Windows tentacle registered and logged as flavor LinuxTentacle, which is misleading — the flavor's registration mechanics are OS-neutral (Windows agents use it too). Rename the flavor to the neutral Tentacle so it no longer distinguishes Windows vs Linux.

• Flavor Id, class (TentacleFlavor), namespace/folder (Flavors/Tentacle), settings (TentacleFlavorSettings), log messages, and both install-snippet builders now use Tentacle.
• The genuinely Linux-specific LinuxTentacleUpgradeStrategy (apt/yum/systemd) keeps its name — it is correctly OS-specific.

Backward compatibility (no breaking)

• LinuxTentacle stays as a flavor alias: ITentacleFlavor gained Aliases, and TentacleFlavorResolver registers each flavor under its id + aliases. Already-deployed agents and old install snippets passing --flavor LinuxTentacle still resolve.
• The legacy LinuxTentacle config section is still bound (alongside the new neutral TentacleFlavor section).
• Flavor resolution is client-side only — the server stores CommunicationStyle (TentaclePolling/Listening), not the flavor — so there is no server/DB change.

Test plan

• TentacleFlavorResolverTests: resolves both Tentacle and LinuxTentacle (alias, case-insensitive) to the same flavor; pins the real TentacleFlavor under both ids
• TentacleFlavorTests: Id == "Tentacle" + Aliases contains LinuxTentacle; flavor settings bind from both the legacy LinuxTentacle: and new TentacleFlavor: config sections (theory)
• Scenario matrix + flavor-coverage contract tests updated to the new id
• Install-snippet builders + register-arg expansion emit/round-trip Tentacle
• Full unit suite (5912/5912), full Tentacle.Tests, full solution build (0 errors); Windows + Linux E2E projects compile