feat(v0.6): internal/agent package — Resolve + BuiltinProfiles

Pure resolution logic combining a workspace's AgentSpec with the
user-level default_agent into a Resolved value carrying Command, Args,
and Env. No I/O — PATH lookup stays in shell.ExecAgent and ctask agents
check, so Resolve is trivially testable and reusable.

BuiltinProfiles enumerates claude and opencode; "custom" is the escape
hatch and requires command. Keep BuiltinProfiles in sync with
workspace.knownAgentTypes and workspace.IsBuiltinAgentType (Task 1).

internal/agent/agent.go

@@ -0,0 +1,81 @@
+package agent
+
+import (
+	"fmt"
+
+	"github.com/warrenronsiek/ctask/internal/workspace"
+)
+
+// Profile describes a built-in agent. The Default command is what ctask
+// invokes when AgentSpec.Command is empty; Type is the canonical name.
+type Profile struct {
+	Type    string
+	Default string
+}
+
+// BuiltinProfiles enumerates the v0.6 built-in agents. "custom" is NOT
+// in this map — it is the escape hatch with no defaults. Keep this in
+// sync with workspace.knownAgentTypes AND workspace.IsBuiltinAgentType
+// (update all three together when a new built-in lands).
+var BuiltinProfiles = map[string]Profile{
+	"claude":   {Type: "claude", Default: "claude"},
+	"opencode": {Type: "opencode", Default: "opencode"},
+}
+
+// IsKnownType reports whether t is "claude", "opencode", or "custom".
+func IsKnownType(t string) bool {
+	if t == "custom" {
+		return true
+	}
+	_, ok := BuiltinProfiles[t]
+	return ok
+}
+
+// Resolved is the launch-ready agent value: a single executable name or
+// path, optional arguments, and an env-var map merged AFTER ctask's own
+// exported vars at launch time.
+type Resolved struct {
+	Type    string
+	Command string
+	Args    []string
+	Env     map[string]string
+}
+
+// Resolve combines a workspace's AgentSpec with the user-level
+// default_agent into a launch-ready Resolved. Resolution rules (per
+// v0.6 spec §5):
+//
+//  1. If spec.Type is empty, fall through to defaultAgent.
+//  2. If the resolved type is "custom", spec.Command is required.
+//  3. Otherwise, the resolved command is spec.Command if set,
+//     else BuiltinProfiles[type].Default.
+//  4. Args and Env are carried verbatim from the spec.
+//
+// Resolve does NOT call exec.LookPath. PATH validation is the launch
+// path's job (shell.ExecAgent fails fast with a diagnostic) and
+// `ctask agents check`'s job. Keeping Resolve I/O-free makes it
+// trivially testable and reusable.
+func Resolve(spec workspace.AgentSpec, defaultAgent string) (*Resolved, error) {
+	typ := spec.Type
+	if typ == "" {
+		typ = defaultAgent
+	}
+	if !IsKnownType(typ) {
+		return nil, fmt.Errorf("agent: unknown type %q (must be claude, opencode, or custom)", typ)
+	}
+
+	cmd := spec.Command
+	if cmd == "" {
+		if typ == "custom" {
+			return nil, fmt.Errorf("agent: type \"custom\" requires command field")
+		}
+		cmd = BuiltinProfiles[typ].Default
+	}
+
+	return &Resolved{
+		Type:    typ,
+		Command: cmd,
+		Args:    spec.Args,
+		Env:     spec.Env,
+	}, nil
+}