feat(daemon): add per-task isolated execution environments

Introduce the `execenv` package that creates isolated working directories
for each agent task. Supports git worktree mode (code tasks) and plain
directory mode (non-code tasks), with `.agent_context/issue_context.md`
injected into the workdir for Claude Code to discover.

Key changes:
- New `server/internal/daemon/execenv/` package (Prepare/Cleanup)
- `runTask()` now creates isolated env instead of using shared reposRoot
- Prompt updated to reference `.agent_context/` files
- Add `WorkspacesRoot` config (default ~/multica_workspaces)
- Add `KeepEnvAfterTask` config for debugging
- Default agent timeout increased from 20min to 2h
- `CompleteTask` now forwards branch name to server

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/internal/daemon/execenv/context.go

@@ -0,0 +1,69 @@
+package execenv
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+)
+
+// writeContextFiles renders and writes .agent_context/issue_context.md into workDir.
+func writeContextFiles(workDir string, ctx TaskContextForEnv) error {
+	contextDir := filepath.Join(workDir, ".agent_context")
+	if err := os.MkdirAll(contextDir, 0o755); err != nil {
+		return fmt.Errorf("create .agent_context dir: %w", err)
+	}
+
+	content := renderIssueContext(ctx)
+	path := filepath.Join(contextDir, "issue_context.md")
+	if err := os.WriteFile(path, []byte(content), 0o644); err != nil {
+		return fmt.Errorf("write issue_context.md: %w", err)
+	}
+	return nil
+}
+
+// renderIssueContext builds the markdown content for issue_context.md.
+// Sections with empty content are omitted.
+func renderIssueContext(ctx TaskContextForEnv) string {
+	var b strings.Builder
+
+	if ctx.IssueTitle != "" {
+		fmt.Fprintf(&b, "# Issue: %s\n\n", ctx.IssueTitle)
+	}
+
+	if ctx.IssueDescription != "" {
+		b.WriteString("## Description\n\n")
+		b.WriteString(ctx.IssueDescription)
+		b.WriteString("\n\n")
+	}
+
+	if len(ctx.AcceptanceCriteria) > 0 {
+		b.WriteString("## Acceptance Criteria\n\n")
+		for _, item := range ctx.AcceptanceCriteria {
+			fmt.Fprintf(&b, "- %s\n", item)
+		}
+		b.WriteString("\n")
+	}
+
+	if len(ctx.ContextRefs) > 0 {
+		b.WriteString("## Context References\n\n")
+		for _, ref := range ctx.ContextRefs {
+			fmt.Fprintf(&b, "- %s\n", ref)
+		}
+		b.WriteString("\n")
+	}
+
+	if ctx.WorkspaceContext != "" {
+		b.WriteString("## Workspace Context\n\n")
+		b.WriteString(ctx.WorkspaceContext)
+		b.WriteString("\n\n")
+	}
+
+	if ctx.AgentSkills != "" {
+		b.WriteString("## Agent Instructions\n\n")
+		b.WriteString(ctx.AgentSkills)
+		b.WriteString("\n")
+	}
+
+	return b.String()
+}