Exports `SanitizePromptText` (and its helpers) from `codersdk` instead
of `coderd/x/chatd`, so API consumers can sanitize prompt text exactly
the way the server stores it.
## Why
coder/terraform-provider-coderd#412 adds a `coderd_chat_system_prompt`
resource whose `system_prompt` attribute compares values by their
sanitized forms (otherwise the trailing newline from
`file("system-prompt.md")` is perpetual drift, since the server stores
the sanitized value). That currently requires a mirrored copy of the
sanitizer in the provider, which can silently rot. Per review there
([discussion](https://github.com/coder/terraform-provider-coderd/pull/412#discussion_r3801110795)),
the sanitizer should be exported from `codersdk` and imported instead.
## What
- `coderd/x/chatd/sanitize.go` → `codersdk/promptsanitize.go` (pure
move; package + doc-comment note about why it lives in codersdk)
- `coderd/x/chatd/sanitize_test.go` → `codersdk/promptsanitize_test.go`
- Callsites updated (`exp_chats.go`, `chatd.go`, `subagent.go`,
`context_prompt.go`); no wrapper left behind, single source of truth
- No behavior change
> Generated by Coder Agents on behalf of @bpmct
Ben Potter
8a7e8d9d5b