chore: Deprecate the `turbo scan` command (#12406)
## Summary
Deprecates `turbo scan` and removes all of its implementation code. The
diagnostics it ran have become irrelevant as `turbo` has evolved:
- **Turbo no longer shells out to Git**, so configuring Git's FS Monitor
daemon provides no benefit to `turbo` performance.
- **The Turborepo daemon is no longer used for `turbo run`**, making the
daemon health check unnecessary.
- **Remote Cache linking should be automatic and frictionless**, not
gated behind an interactive wizard. We have better flows for this now.
- **The LSP check is premature** -- we don't want to actively push users
toward the extension at this stage.
- **Version update checks** are better handled by existing tooling (e.g.
`npx @turbo/codemod update`), not an interactive prompt.
Since every diagnostic `turbo scan` ran is now either obsolete or better
served elsewhere, the command itself no longer provides value.
## What changed
- Deleted the `turborepo-diagnostics` crate and the `commands/scan.rs`
implementation.
- `turbo scan` is now hidden from `--help` and prints a deprecation
warning if invoked.
- Documentation updated: reference page shows a deprecation callout,
reference index card marked deprecated, and `turbo scan` added to the
[deprecated list on the support policy
page](https://turborepo.dev/docs/support-policy#deprecated).
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Co-authored-by: Vercel <vercel[bot]@users.noreply.github.com>
