chore: finish sprout->buzz rename in relay env vars and public-facing strings

[SeanGearin]

Problem

The block/sprout -> block/buzz rename left residue in public-facing strings and, more importantly, in three relay environment variables that still use the old SPROUT_ prefix while every other relay knob is BUZZ_-prefixed:

• SPROUT_MAX_NOT_BEFORE_DELTA — read in two places (crates/buzz-relay/src/nip11.rs and crates/buzz-relay/src/handlers/ingest.rs); it is both advertised in the NIP-11 relay information document (limitation.max_not_before_delta) and enforced on NIP-ER reminder ingest, so the relay literally self-identifies with a stale config surface.
• SPROUT_REMINDER_SCHEDULER_INTERVAL_SECS / SPROUT_REMINDER_SCHEDULER_BATCH_LIMIT — read in crates/buzz-relay/src/main.rs for the NIP-ER push scheduler.

Plus cosmetic residue: the NIP-AO reference-implementation link and a source comment still point at github.com/block/sprout, a doc comment names the pre-rename sprout-dev-mcp crate, and one buzz-acp test uses an internal pre-rename staging hostname as its example URL.

Fix

• Env vars are read as BUZZ_* first with a fallback to the legacy SPROUT_* name, so existing deployments that persisted the old names keep their configured horizon/scheduler tuning — no behavior change for anyone. Comments at each read site note the legacy fallback. A follow-up can drop the fallback once deployments have migrated.
• .github/workflows/ci.yml now sets BUZZ_REMINDER_SCHEDULER_INTERVAL_SECS=1 (2 lines), so CI's e2e relay jobs exercise the new name end-to-end.
• docs/nips/NIP-AO.md reference-implementation link and a handlers/side_effects.rs comment updated to github.com/block/buzz (GitHub redirects the old URLs, but the text no longer names the dead repo).
• crates/buzz-agent/src/config.rs doc comment: sprout-dev-mcp -> buzz-dev-mcp (the crate's actual name).
• crates/buzz-acp/src/config.rs test: wss://sprout-oss.stage.blox.sqprod.co -> wss://relay.example.com, matching the sibling tests' example hosts and removing an internal hostname from the public repo.
• crates/buzz-test-client/tests/e2e_event_reminder.rs doc comment updated to the new scheduler var name.

Deliberately NOT touched (intentional references): squareup/sprout-* internal-repo references in AGENTS.md/CONTRIBUTING.md; legacy-migration/cleanup code in scripts/dev-setup.sh, scripts/instance-env.sh, scripts/reset-desktop-dev-state.sh (they migrate/clean old sprout-named state by design); scripts/check-pr-image-urls.sh (its pattern intentionally matches the old internal media host); the sprout_ref Buildkite input in RELEASING.md and workflows (an internal pipeline's real input name); the SPROUT_UPDATER_PUBLIC_KEY secret fallback in release.yml (already a deliberate compat fallback); the historical crates/sprout-relay/... pre-rewrite reference in conformance_multitenant.rs; Cargo.toml repository URLs (covered by PR #2519) and the sprout-cli skill dirs (covered by PR #2525); CHANGELOG/git history.

Test evidence

• cargo fmt -p buzz-relay -p buzz-acp -p buzz-agent -p buzz-test-client --check — clean.
• cargo check -p buzz-relay -p buzz-acp -p buzz-agent — clean.
• cargo clippy -p buzz-relay -p buzz-acp -p buzz-agent -- -D warnings — clean.
• No unwrap()/expect() introduced; env reads keep the existing .ok().and_then(...).unwrap_or(default) shape.

Links

• Related prior renames: fix(desktop): rename SPROUT_ env vars to BUZZ_ for child agent processes #971 (desktop SPROUT_ env vars), refactor: rename sprout backend to buzz #958, fix(release): update repository name check from block/sprout to block/buzz #1012.
• Complementary open PRs (no overlap): chore: update Cargo repository URLs from block/sprout to block/buzz #2519 (Cargo repository URLs), chore(skills): rename stale sprout-cli skill dir to buzz-cli #2525 (sprout-cli skill dirs).