[B3] Channel write scope (no longer global)

[radical]

Track B implementation. Depends on Q3 (#34).

UpdateCommand.ExecuteSelfUpdateAsync (around line 329) writes channel to the global ~/.aspire/aspire.config.json. In SxS scenarios this corrupts other installs (W34, W35).

After Q3 is decided:

• Implement the channel scope (per-route, hybrid, or whatever lands).
• For winget/brew routes, suppress the channel write entirely (channel doesn't apply — Q7).
• Add aspire config set channel --scope flag if hybrid model wins.

Code touched

• src/Aspire.Cli/Commands/UpdateCommand.cs:329-337
• src/Aspire.Cli/Configuration/* (config layer)

Scenarios advanced

S2.6, S4.1, S4.2, S4.3.

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Part of: #29