docs: publish Tend Compose continuity evidence

Export the reviewed allowlisted snapshot from private source commit 07c1655921f21ee5e4fc4d85639d199e8867b17d. This records the Docker Compose activation, schema-compatible rollback, and stateful migration resource findings from Observatory Preview 19 dogfooding.

AI-Assisted: OpenAI Codex
Signed-off-by: Cole Speelman <crspeelman@gmail.com>

internal/process/run.go

@@ -0,0 +1,72 @@
+// SPDX-License-Identifier: AGPL-3.0-only
+
+package process
+
+import (
+	"bytes"
+	"context"
+	"fmt"
+	"os"
+	"os/exec"
+	"sort"
+	"strings"
+)
+
+const maxOutput = 4 << 20
+
+type Runner interface {
+	Run(ctx context.Context, dir string, env map[string]string, name string, args ...string) ([]byte, error)
+}
+
+type ExecRunner struct{}
+
+func (ExecRunner) Run(ctx context.Context, dir string, env map[string]string, name string, args ...string) ([]byte, error) {
+	cmd := exec.CommandContext(ctx, name, args...)
+	cmd.Dir = dir
+	cmd.Env = mergeEnv(os.Environ(), env)
+	var output limitedBuffer
+	cmd.Stdout = &output
+	cmd.Stderr = &output
+	err := cmd.Run()
+	if err != nil {
+		return output.Bytes(), fmt.Errorf("%s failed: %w: %s", name, err, strings.TrimSpace(output.String()))
+	}
+	return output.Bytes(), nil
+}
+
+type limitedBuffer struct{ bytes.Buffer }
+
+func (b *limitedBuffer) Write(p []byte) (int, error) {
+	written := len(p)
+	remaining := maxOutput - b.Len()
+	if remaining > 0 {
+		if len(p) > remaining {
+			p = p[:remaining]
+		}
+		_, _ = b.Buffer.Write(p)
+	}
+	return written, nil
+}
+
+func mergeEnv(base []string, extra map[string]string) []string {
+	values := make(map[string]string, len(base)+len(extra))
+	for _, pair := range base {
+		key, value, ok := strings.Cut(pair, "=")
+		if ok {
+			values[key] = value
+		}
+	}
+	for key, value := range extra {
+		values[key] = value
+	}
+	keys := make([]string, 0, len(values))
+	for key := range values {
+		keys = append(keys, key)
+	}
+	sort.Strings(keys)
+	out := make([]string, 0, len(keys))
+	for _, key := range keys {
+		out = append(out, key+"="+values[key])
+	}
+	return out
+}