feat: add retrospective MCP tool

Adds internal/skills/retrospective/ — an MCP skill that reads a session
log and dispatches a worker subprocess (via ExecutorFn) to identify
learnings and write them to the brain. Follows the same executor pattern
as the TDD skill.

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

internal/skills/retrospective/skill.go

@@ -0,0 +1,50 @@
+// internal/skills/retrospective/skill.go
+package retrospective
+
+import (
+	"context"
+	"encoding/json"
+
+	iexec "github.com/mathiasbq/supervisor/internal/exec"
+	"github.com/mathiasbq/supervisor/internal/registry"
+)
+
+// ExecutorFn allows injecting a test double for the subprocess executor.
+type ExecutorFn func(ctx context.Context, req iexec.Request) (iexec.Result, error)
+
+// Config holds retrospective skill configuration.
+type Config struct {
+	SkillPrompt  string     // content of retrospective.md
+	DefaultModel string     // model to use when not specified in args
+	SessionsDir  string     // path to brain/sessions/
+	ExecutorFn   ExecutorFn // injected executor
+}
+
+// Skill implements registry.Skill for the retrospective tool.
+type Skill struct {
+	cfg Config
+}
+
+// New constructs a retrospective Skill.
+func New(cfg Config) *Skill { return &Skill{cfg: cfg} }
+
+// Name returns the skill name.
+func (s *Skill) Name() string { return "retrospective" }
+
+// Tools returns the MCP tool definitions.
+func (s *Skill) Tools() []registry.ToolDef {
+	return []registry.ToolDef{
+		{
+			Name:        "retrospective",
+			Description: "Run a retrospective on a completed session. Reads the session log, identifies novel learnings, and writes structured entries to the brain for ingestion. Call at the end of each coding session.",
+			InputSchema: json.RawMessage(`{
+				"type": "object",
+				"required": ["session_id"],
+				"properties": {
+					"session_id": {"type": "string"},
+					"model":      {"type": "string"}
+				}
+			}`),
+		},
+	}
+}