Implement `exec` subcommand

[mikkeldamsgaard]

Summary

Implement the exec subcommand for running arbitrary commands with structured logging.

Requirements

• Execute command via execve (no shell) using -- separator
• Capture stdout/stderr and print with timestamps
• Optional JSON log output via --json
• Forward the child process exit code
• Support --workdir for setting the working directory of the child process
• Intended for use with custom images, add a section to usage.md about how to build an image with this as base with a set of commands that can be executed this way, it could be openssl to seed the container with a private key.
• Add an additional docker file and image that builds jq and yq into the image. Call it initium-jyq. Update release.yml

Acceptance Criteria

• internal/cmd/exec.go with full implementation
• internal/cmd/exec_test.go with tests for: success, failure exit codes, -- parsing, stdout/stderr capture, JSON logging
• Registered in cmd/initium/main.go
• docs/usage.md updated with exec section
• CHANGELOG.md updated
• All tests pass

[mikkeldamsgaard]

Implemented in commit 899c0de. All acceptance criteria met:

• ✅ internal/cmd/exec.go with full implementation (execve, stdout/stderr capture, exit code forwarding, --workdir, --json)
• ✅ internal/cmd/exec_test.go with 11 tests (no args, success, exit code 42, stdout/stderr, JSON output, workdir, command not found, multiple args, exit code 1, start message, help output)
• ✅ Registered in cmd/initium/main.go
• ✅ docs/usage.md updated with exec section + custom image building guide + initium-jyq documentation
• ✅ Dockerfile.jyq added for initium-jyq variant with jq and yq
• ✅ release.yml updated to build and push initium-jyq image
• ✅ CHANGELOG.md updated
• ✅ Refactored runCommand in migrate.go: extracted shared helpers (newExecCommand, executeAndStream) — all existing migrate/seed tests still pass
• ✅ All tests pass (go test ./... -> all ok)
• ✅ Code formatted (gofmt)