chore: harden repository security

[cmilesio]

What

Cover all three Execx modules with reproducible source and dependency security checks.

Control	Scope
Organization-managed CodeQL	All Go source
govulncheck	All three module graphs
Gitleaks	Full repository history
CycloneDX	Three canonical JSON library SBOMs with test dependencies
Dependabot	Every Go module and GitHub Actions
Dependency graph	Root and examples select golang.org/x/sys 0.44

The examples continue resolving the root locally through equivalent ./.. syntax, preventing ambiguous SBOM components. The minimum Go version remains 1.25. There is no command execution API, process behavior, configuration, or persisted-data change.

Why

Execx sits on a process-execution boundary. This PR does not expand that boundary; it removes the current system-call dependency advisory and makes future changes easier to audit with complete source, dependency, history, and inventory evidence.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!

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