docs(desktop): add architecture migration issue block

[Whiteks1]

Summary

• Add the top-level Desktop migration issue block as real slices instead of a micro-backlog.
• Mirror each migration slice into .agents/tasks/ and record ordering plus dependencies.

Issue

• N/A. This planning PR creates the migration issue block and does not close an implementation issue.

Why

• The desktop migration needed an explicit slice sequence before starting the architecture transition.
• This reduces improvisation and keeps the workflow aligned with one slice per issue, branch, and PR.

Scope

This PR does not:

• Add unrelated feature work.
• Expand scope beyond the slice.
• Introduce a second source of truth for the same content or behavior.

Validation

Validated with:

• Planning and documentation diff only.
• GitHub issues #350 through #359 created.
• Diff limited to .agents/.

Duplication Check

• I searched for existing docs, surfaces, or contracts covering the same idea.
• I removed or replaced stale copies instead of keeping both versions.
• I did not introduce alternate lockfiles, package managers, or build paths.

Compatibility / Risk

• No runtime or renderer compatibility impact; this is a planning-only PR.
• Risk is limited to issue sequencing being wrong, which is why dependencies are recorded explicitly.

Notes

• This PR only aligns docs and planning continuity.
• It does not implement any runtime, renderer, or architecture behavior change.

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