feat(tdd): inject session history into green and refactor worker prompts

Adds SessionsDir to tdd.Config, session_id to tool input schemas, and a
prependHistory method that reads the session JSONL log and prepends a
formatted history block to the task prompt before worker invocation.

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

internal/skills/tdd/skill.go

@@ -16,6 +16,7 @@ type Config struct {
 	SkillPrompt  string
 	ExecutorFn   ExecutorFn // nil = no executor (tests that don't reach execute())
 	DefaultModel string
+	SessionsDir  string // optional: path to brain/sessions/ for history injection
 }
 
 type Skill struct {
@@ -63,6 +64,7 @@ func (s *Skill) Tools() []registry.ToolDef {
 					"test_path":    strProp,
 					"model":        strProp,
 					"test_cmd":     strProp,
+					"session_id":   strProp,
 				},
 			),
 		},
@@ -77,6 +79,7 @@ func (s *Skill) Tools() []registry.ToolDef {
 					"impl_path":    strProp,
 					"model":        strProp,
 					"test_cmd":     strProp,
+					"session_id":   strProp,
 				},
 			),
 		},