chore: pre-rename "sprout" identifiers are still load-bearing (Cargo.toml repository URL, SPROUT_* env vars)

Observed at commit 63496cc1d4c6f1b7c613801bdcc694169dcf391a (v0.5.2-5-g63496cc1). Related to the now-closed #2557 (AGENTS.md sprout references) but these two are in code/metadata, not docs:

**1. Workspace metadata points at the old repo**

`Cargo.toml:39`:

```toml
repository = "https://github.com/block/sprout"
```

Anything consuming workspace metadata (crates.io publishing, cargo-generated docs links, SBOM tooling) inherits the wrong URL.

**2. Three `SPROUT_`-prefixed env vars are live runtime config**

- `SPROUT_MAX_NOT_BEFORE_DELTA` — read in `crates/buzz-relay/src/handlers/ingest.rs` (~L1338) and advertised in NIP-11 as `max_not_before_delta`
- `SPROUT_REMINDER_SCHEDULER_INTERVAL_SECS`
- `SPROUT_REMINDER_SCHEDULER_BATCH_LIMIT`

Every other tunable uses the `BUZZ_` prefix, so operators grepping their deployment for `BUZZ_` config will not find these — the reminder/scheduling knobs silently look like they don't exist. (We hit exactly this while auditing a deployment's env.)

**Suggested fix:** update the `repository` field, and either rename the env vars to `BUZZ_` with the `SPROUT_` names kept as deprecated fallbacks for a release, or at minimum document the `SPROUT_` names wherever env config is documented.

Found while building code-derived documentation of the platform (with AI assistance); happy to send a PR for the fallback-alias approach if useful.