fix(desktop): make starter profiles optional and removable

[JackHunzicker]

Summary

Starter personas and the built-in Welcome Team are currently forced into every user's inventory and re-created after removal. This PR keeps the templates available by explicit choice instead.

• New built-in personas start inactive; existing activation choices and customizations are preserved.
• The startup fold preserves inactive/customized built-ins instead of discarding their opt-out state.
• Missing Welcome Teams are not automatically recreated, and onboarding respects absent teams or inactive starters instead of activating and launching them again.
• The Agents screen gets an explicit Starter templates menu. Stock profiles and the Welcome Team can be removed through the ordinary delete controls and stay removed across reload.
• Removing a local built-in team does not queue kind:30176 or catalog deletion events (built-in templates are never owner-published, so a synchronized tombstone could delete another device's customized copy). Owner-authored custom teams keep normal synchronized deletion. Existing dependency/reference guards and same-named custom teams are preserved.

Independent of #7419. Based on 3c7f288c60d67df78577b237e27c3dfc8831aaa1. No change to agent runtime/harness configuration, no second profile store, and the stock template definitions themselves are not removed.

Related issue

Fixes #5553 (allow hiding or disabling built-in personas and the Welcome Team). Adjacent, not duplicates: #5979 (channel-membership repair after a completed kickoff; this PR touches welcomeKickoff.ts on one line and should compose with it), #5319 / #6211 (per-device duplicate agents).

Testing

• Native desktop library: 3,013 passed, 0 failed, 15 ignored (cargo test --manifest-path desktop/src-tauri/Cargo.toml --lib --locked). Native regressions cover local-only built-in deletion, custom-team tombstone retention through the same production seam, opt-out survival through the real startup migration/fold, and no automatic Welcome Team re-creation.
• Desktop JS: 6,452 passed, 0 failed (pnpm test, both globs).
• pnpm build:e2e (TypeScript check + mock-bridge build) passed.
• Browser (Playwright smoke, tests/e2e/removable-defaults.spec.ts): 2 passed: starter templates are opt-in and can be removed again; deleting the built-in Welcome Team leaves a custom look-alike team intact.
• Rust fmt, Biome on touched frontend files, git diff --check, and just file-size-check pass.

Verified on Windows (x86_64-pc-windows-msvc). The full just ci on this host surfaced a set of pre-existing Windows-only Clippy/test-fixture issues on main that are unrelated to this change; I have those fixed locally and will open them as a separate PR so this one stays scoped.

Screenshots (mock bridge):

Starter templates control, no forced profiles	Delete on a starter profile

Stays removed after reload	Custom look-alike team survives built-in team deletion

[github-actions]

🔐 Codex Security Review

Status: review required for the current range.

The current range is 3c7f288c60d67df78577b237e27c3dfc8831aaa1...bdea3002df41d33af2b69b2a7a1b2fddbbdb1ce8.
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[coreyone]

Support. Optional, removable starter profiles reduce unwanted local agents without changing relay identity cleanup. This complements #7465 by covering built-in profiles separately.

[cogpros]

+1 from a user hitting #6914 on v0.5.24. This PR would fix it.

One case worth covering in review or tests: customized built-in personas get cloned into a second community's Welcome Team. I had edited builtin:fizz and builtin:honey (renamed them and gave them custom system prompts) in my first community. Joining a second community seeded Fizz and Honey instances there that carry those custom prompts, and it started them. Details are in #6914.

With this PR, "Missing Welcome Teams are not automatically recreated" and "onboarding respects inactive starters" should stop the respawn. It isn't clear to me whether a customized, active built-in would still be copied into a new community on first onboarding. That may deserve a guard, such as seeding from the stock definition or skipping customized built-ins.