feat: RELAY-conformant cpp-rcp CLI (§11/§12 version/capabilities/status)

[SoundMatt]

Summary

Adds the cpp-rcp CLI binary so relay conform / relay report (RELAY tool v0.7/v0.8) can validate cpp-RCP against the embedded §12 JSON schemas. This is the v0.3 CLI-conformance follow-up flagged when adopting the spec (issue #11).

Implements the three mandatory commands (§11.1):

Command	Output	Schema
version [--format text|json]	§12.1 version document	cli-version.json
capabilities	§12.2 capabilities document (always JSON)	cli-capabilities.json
status [--format text|json]	§12.3 status document	cli-status.json

Exit codes per §11.3: 0 success, 2 invalid args.

Design

• include/rcp/cli.hpp — rcp::cli::run(args, out, err) holds all logic in the header so it's unit-testable without spawning a subprocess. JSON is hand-written to exact, fixed shapes (no JSON dependency added).
• cli/main.cpp — thin argv→run() wrapper.
• include/rcp/version.hpp — rcp::kVersion = "0.3.0".
• CMake — -DRELAY_BUILD_CLI=ON builds the cpp-rcp binary (§13.2 lowercase name); project version bumped 0.1.0 → 0.3.0.

Schema fidelity

All three documents are field-exact against the schemas (which set additionalProperties: false):

$ cpp-rcp version --format json
{"tool":"cpp-rcp","protocol":"RCP","protocol_int":5,"version":"0.3.0","spec_version":"0.3","language":"cpp","runtime":"clang-21"}
$ cpp-rcp capabilities
{"kind":"capabilities","tool":"cpp-rcp","version":"0.3.0","spec_version":"0.3","commands":["version","capabilities","status"],"transports":["mock","sim","shmem"],"features":["loaning"],"interfaces":["Node","Caller"],"optional_interfaces":["LoaningController"],"adapt":true}
$ cpp-rcp status --format json
{"protocol":"RCP","tool":"cpp-rcp","version":"0.3.0","healthy":true,"connected":false,"endpoint":"","details":{}}

Validated with Python: valid JSON, no extra keys, all required keys present for all three.

Test plan

• 8 new rcp_cli conformance tests; 28/28 CTest cases pass locally
• Binary exercised directly (exit codes 0/0/0, unknown cmd → 2)
• CI matrix green (now builds -DRELAY_BUILD_CLI=ON on all 5 platforms)