Clean up agent guide and remove unused Core package references

[siewers]

Fixes inconsistencies and redundancies in AGENTS.md, and makes the code match its dependency rule.

Remove unused package references from Core (separate commit)

• Core referenced CommunityToolkit.Mvvm and Microsoft.Extensions.DependencyInjection, but no Core code uses either. Removed both; UI already referenced Mvvm and now references DI directly instead of getting it through Core.
• Core's only package dependency is now Zeroconf. This also prevents the NU1605 downgrade failures seen in Bump CommunityToolkit.Mvvm from 8.4.0 to 8.4.1 #17 and Bump CommunityToolkit.Mvvm from 8.4.0 to 8.4.2 #33, which came from Mvvm being referenced by both projects.

AGENTS.md

• Reworded the "dependency-free beyond the BCL" rule, which Core didn't follow: Core's only package is Zeroconf, and new ones must be small abstraction packages, never UI, MVVM or DI frameworks.
• Clarified that CommunityToolkit.Mvvm is used by UI, not Core.
• Device testing: try the last known IP first and ask only if it fails, instead of always asking and also storing an IP.
• Merged the "generated by AI" and Co-Authored-By rules into one "no AI attribution" rule, and moved the emoji rule out of "Code comments" into a General section, since it covers more than comments.
• Fixed heading levels (# for the title).
• Added sections on building and testing, line endings, pull request labels for release notes, the SemVer release process, and the macOS "No route to host" adb workaround (Connecting fails with "No route to host" when the adb server lacks macOS local network access #35).

Builds without warnings, 57 tests pass.