Add Cloud Agent dev environment install script

[TomKaltz]

Summary

Adds tools/linux/cloud-agent-install.sh, an idempotent bootstrap for the CasparCG Server development environment. It installs the build toolchain + system dependencies and performs a full CMake/Ninja build via the recommended system-CEF path (ppa:casparcg/ppa).

This is the human-runnable equivalent of the install script now backing the Cursor Cloud Agent dashboard-managed environment (which uses an inline copy of the same commands so it does not depend on this unmerged file).

Why

The dashboard-managed dev environment previously lacked build dependencies (no Boost/FFmpeg/Ninja) and its recurring environment build was failing. This script makes the environment build the server end to end on a clean Ubuntu 24.04 (noble) image and encodes two fixes discovered during validation:

• Pin the GCC toolchain (CC=gcc/CXX=g++, plus -DCMAKE_C(XX)_COMPILER). The base image points the c++ alternative at clang, whose GCC autodetection selects a gcc dir without libstdc++ dev files and fails to link (-lstdc++).
• Add -Wl,--copy-dt-needed-entries. libavdevice pulls in libcaca, which needs libncursesw symbols; modern ld defaults to --no-copy-dt-needed-entries and leaves those transitive symbols unresolved when linking shell/casparcg.

Validation

• Ran the script end to end on a clean noble VM: dependencies install, CMake configures (Build Version 2.5.0 Dev), and shell/casparcg links successfully (ldd reports no missing libraries; the binary starts and parses args).
• Verified idempotence: a second run reuses the existing build cache with no recompilation.
• Verified the equivalent config in a fresh Cloud Agent environment build (bld-20260805-cbafc016...): fresh checkout of master + full install → [129/129] Linking CXX executable shell/casparcg, build SUCCEEDED.

Note: the environment is dashboard-managed (DB). Merging this PR keeps the committed script in sync, but the saved environment is updated separately via Save in the Cloud Agent environment panel.