docs: add contributor guidance and refresh README

[edsiper]

Summary

• rewrite the README with current repository URLs, pinned submodule setup, out-of-source CMake commands, verified options, tests, installation, dependencies, and contribution links
• add AGENTS.md as the canonical, vendor-neutral repository operating guide
• add a one-line CLAUDE.md adapter that imports the canonical guide
• document investigation, bug-fix, code-review, and cross-repository workflows
• add small reusable CMake/CTest build, test, and verification wrappers

Why

The README referenced the former repository location, advanced submodules with --remote, and assumed an existing in-tree build directory. CTraces also had no repository-owned entry point for coding agents or canonical local verification command.

This change makes public setup instructions reproducible and records verified architecture, ownership, compatibility, dependency, and validation rules in normal repository documentation without adding vendor-specific policy.

Impact

Users get current build, test, and installation instructions. Codex CLI, Claude Code, human contributors, CI, and future tools can follow the same repository-owned guidance and execute the same validation wrappers. Production source, public APIs, vendored code, generated files, submodules, tests, and CI behavior are unchanged.

Validation

• README link validation
• README CMake configure and build commands
• README CTest command: 4/4 tests passed
• staged cmake --install validation
• bash -n scripts/agent-build.sh scripts/agent-test.sh scripts/agent-verify.sh
• ShellCheck on all three scripts
• targeted build and ctr-test-span: 1/1 passed
• scripts/agent-verify.sh: 4/4 tests passed
• documentation link validation
• verified CLAUDE.md contains only @AGENTS.md
• git diff --check