docs: clarify /dashboard?tab= links are stable, not legacy (#430)

## Summary

Adds documentation comments clarifying that the `/dashboard?tab=<tab>`
links are **not legacy** and must keep working forever. These links are
stable public entrypoints crosslinked from emails, docs, and the CLI, so
removing tabs or changing the redirect contract would break those
external references.

No behavior change — comments only.

- `src/configs/dashboard-tab-url-map.ts`: doc comment on `TAB_URL_MAP`
explaining the links are stable public entrypoints and must not be
removed.
- `src/app/dashboard/route.ts`: doc comment on the `GET` handler
describing the `/dashboard?tab=keys` → `/dashboard/<team-slug>/keys`
redirect behavior and that it must keep working.

Context: Slack thread where Ben confirmed these are not legacy; Jakub
asked to add a comment so they aren't accidentally removed.

Link to Devin session:
https://app.devin.ai/sessions/a908d9ec7fd24404ab77b44622e4f716

Co-authored-by: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>
Co-authored-by: jakub@e2b.dev <jakub@e2b.dev>

Author: devin-ai-integration[bot]

src/app/dashboard/route.ts

@@ -18,6 +18,14 @@ function getTabRedirectPath(tab: string | null, teamSlug: string) {
   return PROTECTED_URLS.SANDBOXES(teamSlug)
 }
 
+/**
+ * `/dashboard?tab=<tab>` resolves the user's default team and redirects to the
+ * team-scoped page (e.g. `/dashboard/keys` -> `/dashboard/<team-slug>/keys`).
+ *
+ * This is NOT legacy: these links are stable public entrypoints crosslinked
+ * from emails, docs, and the CLI, so this route must keep working. See
+ * `TAB_URL_MAP` for the supported tabs.
+ */
 export async function GET(request: NextRequest) {
   const searchParams = request.nextUrl.searchParams
   const tab = searchParams.get('tab')

src/configs/dashboard-tab-url-map.ts

@@ -1,5 +1,13 @@
 import { PROTECTED_URLS } from './urls'
 
+/**
+ * Maps `/dashboard?tab=<tab>` query values to team-scoped dashboard URLs.
+ *
+ * These `?tab=` links are NOT legacy: they are stable public entrypoints used
+ * to crosslink into the dashboard from outside (emails, docs, the CLI, etc.).
+ * They must keep working forever — do not remove tabs or change this contract
+ * without coordinating with the places that link here.
+ */
 export const TAB_URL_MAP: Record<string, (teamSlug: string) => string> = {
   sandboxes: (teamSlug) => PROTECTED_URLS.SANDBOXES(teamSlug),
   templates: (teamSlug) => PROTECTED_URLS.TEMPLATES(teamSlug),