Add C implementations of device-connect-edge and -agent-tools

[atsyplikhin]

Summary

Two new packages porting Device Connect to C — the device side and the external-agent side — mirroring the Python device-connect-edge / device-connect-agent-tools. Built in the same spirit as the MHP C SDK wire node, reusing its proven JSON / JSON-RPC / NATS-transport / security modules.

packages/device-connect-edge-c/ (libdc_edge.a)

• driver (dc/driver.h): register @rpc-style functions by name, advertise events, set identity/status. DC RPC semantics — method is the function name, params are the args, the reply carries the raw return value as result.
• runtime (dc/runtime.h): connect over NATS (cnats), serve device-connect.{tenant}.{id}.cmd, registerDevice to the registry, heartbeat every device_ttl/3 to hold the lease (device shows online), answer requestRegistration, re-register on reconnect, plus invoke_remote (D2D) and emit (events).
• credentials: consumes the portal's native *.creds.json directly (auto-converts to nsc-chained for cnats; auto-detects device_id/tenant) or an nsc *.creds.
• example: temp_sensor.c (get_reading/set_target + a reading_changed event).

packages/device-connect-agent-tools-c/ (libdc_agent_tools.a)

• the four meta-tools over the portal HTTP API (libcurl): describe_fleet, list_devices, get_device_functions, invoke_device; responses parsed to JSON.
• example: dc_agent CLI (fleet/list/functions/invoke).

Testing

• Unit: edge 4/4 (driver capabilities/identity/status, dispatch, JSON-RPC shapes), agent-tools 3/3 (env resolution, portal-response parsing). make test in each package.
• Live end-to-end against a real DC portal (tenant alpha): 3 C edge devices provisioned → registered → devices_online: 3 sustained by heartbeat → discovered by the C agent-tools (fleet/list/functions) → invoked C→C (set_target→{"ok":true}, get_reading→{"temp_c":…} across all three, ~170 ms), with -32601/-32602 device errors propagating through the portal envelope.
• CI (.github/workflows/c-sdk.yml): builds + unit-tests both packages on Linux and macOS (cnats from source on Linux / brew on macOS; libcurl).

Dependencies

• edge-c: the NATS C client (cnats). agent-tools-c: libcurl. Both C11, ASCII-only, Apache-2.0 with per-file SPDX headers.

Scope / not yet

• edge transport is NATS (the dc/transport.h vtable leaves room for MQTT/Zenoh); @periodic/@on/D2D presence-collector and a few niceties are not yet wired (portal-mode register+heartbeat+cmd+invoke_remote is complete).
• agent-tools covers the four core tools; event streaming and the Strands/LangChain/MCP adapters from the Python package are not ported.

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