[FEATURE] Buzz: install.sh soft-install helper for buzz-cli

[muddlebee]

Split out of #4762 (two-way transport) to keep that scope focused on the gateway transport itself.

Problem

buzz-cli (crate buzz-cli, binary buzz) is not published to any package registry — installing it means cargo install --path crates/buzz-cli from a clone of https://github.com/block/buzz. Most users won't do that by hand, so Buzz stays effectively opt-in-only for people who already have a Rust toolchain.

Proposed solution

Add a soft-install helper to install.sh, mirroring the existing ensure_github_cli() pattern:

• If cargo is present: install buzz-cli automatically (shallow clone + cargo install --path crates/buzz-cli, or the published-crate path if/when Block ever publishes one).
• If cargo is absent: print manual instructions and continue — never fail the overall opensre install over an optional integration.

Out of scope here: bundling a prebuilt buzz binary into OpenSRE's own release pipeline (would mean cross-compiled artifacts per platform in .github/workflows/release.yml — a bigger, separate decision or bundling upstream release binaries if/when Block starts publishing them).

References

• [FEATURE] Buzz (block/buzz) integration #4190 — original Buzz feature request
• feat(integrations): add Buzz (block/buzz) delivery integration #4756 — delivery-tier PR (soft dependency via BUZZ_PATH, doctor check)
• [FEATURE] Buzz: two-way communication (gateway transport) #4762 — two-way transport (this was originally scoped there, split out to keep that PR focused)

[arulgoyal]

hey @muddlebee can i work on this ?