[v3.1 PR-7] Stop writing global identity-channel; reseed project-channel default; remove dead Acquisition stubs

[radical]

Design changes: 8a, 8b, 14 — see agreed-design-v3.md §4 Required changes.
Tracker: #16737

"Change N" numbers below refer to entries in the design doc's §4 table, not GitHub issues.

Scope

Change 8a — stop writing global identity-channel

• get-aspire-cli.{sh,ps1}#save_global_settings no longer writes ~/.aspire/aspire.config.json#channel.
• UpdateCommand --self no longer writes it.
• The legacy globalsettings.json migration in Program.cs:156-182 drops channel on import.
• Per delivery-plan §1 item 4, schema field stays one release for compat.

Change 8b — reseed project-channel default

• Scaffolding (ScaffoldingService.cs:64-66, :199), starter template factories (CliTemplateFactory.{Go,Python,TypeScript}StarterTemplate.cs), and GuestAppHostProject use baked AspireCliChannel from PR-1 ([v3.1 PR-1] Bake AspireCliChannel into binary + ChannelReader + build-side verification #16738) as the default seed instead of the global config or a free-form prompt.
• The 7 project-channel readers (AddCommand.cs:110-111, PrebuiltAppHostServer.cs:343-344, DotNetBasedAppHostServerProject.cs:327-328, etc.) do not move.

Change 14 — cleanup

• Remove dead src/Aspire.Cli/Acquisition/*.cs stubs (IdentityChannelReader, IdentityManifest.Channel, etc.).
• Remove [ActiveIssue] gates as the corresponding tests in Aspire.Acquisition.Tests (Add comprehensive test coverage for CLI acquisition scripts #15995, merged) are rewritten.

Acceptance

• ~/.aspire/aspire.config.json#channel is no longer written by any code path.
• A fresh project scaffolded by a daily CLI defaults to the daily channel without prompting.
• aspire add continues to honor a project's aspire.config.json#channel if explicitly set.
• No [ActiveIssue] gates remain in Aspire.Acquisition.Tests for v3.1 stub-replaced tests.

Risk

Low — but call out the scaffolding UX change in release notes. The prior prompt was already fragile.

Open decisions blocking

• Tracker Aspire CLI acquisition coherence (tracking) #16737 delivery-plan §1 item 4: deprecation timing (stop-writing release; field-deletion release)

Split fallback

If review burden demands: PR-7a (Changes 8a + 8b deprecation/reseed) → PR-7b (Change 14 stub cleanup).

Dependencies

• PR-1 ([v3.1 PR-1] Bake AspireCliChannel into binary + ChannelReader + build-side verification #16738) — baked channel for seeding
• PR-4 ([v3.1 PR-4] RouteDetector + path resolution + UpdateCommand --self policy gating (★ ships bug fixes) #16741) — so --self no longer writes global

[radical]

Superseded by the v3 PR grouping (see #16737). The work tracked here is folded into the new PR 4 — Schema cleanup.