plugin: handle --version/--help in protoc-gen-buffa{,-packaging}

[iainmcgin]

Both plugin binaries now respond to --version / -V and --help / -h instead of blocking on stdin (the previous behavior when invoked from a TTY: read_to_end waits for Ctrl-D, then dumps a near-empty binary CodeGeneratorResponse). Any other CLI argument prints a "this is a protoc plugin" hint to stderr and exits 2.

protoc and buf invoke plugins with no argv (parameters travel inside CodeGeneratorRequest.parameter on stdin), so the stdin protocol path is unchanged.

$ protoc-gen-buffa --version
protoc-gen-buffa 0.4.0
$ protoc-gen-buffa-packaging -V
protoc-gen-buffa-packaging 0.4.0

Also drops the redundant --plugin=protoc-gen-buffa from the canonical protoc usage example (protoc finds protoc-gen-* on PATH automatically; --plugin= is only needed to point at a non-PATH binary).

Testing

Integration tests in each crate's tests/cli.rs invoke the built binary via CARGO_BIN_EXE_* and cover --version, -V, --help/-h, and the unrecognized-arg path (asserting exit code 2).

Semver

Additive CLI behavior on the two binary crates; no library API change. Suitable for 0.4.1.

[github-actions]

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