Add knowledge port and single-file adapter

The knowledge.Source port answers "who am I; what are my priorities?".
The FileSource adapter reads one profile file (path selectable per call),
treats a missing/blank file as absent (not an error), and caps the text.
Leaf package mirroring tasks.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/knowledge/file_test.go

@@ -0,0 +1,98 @@
+package knowledge
+
+import (
+	"context"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+func TestLoadPresentFile(t *testing.T) {
+	dir := t.TempDir()
+	p := filepath.Join(dir, "knowledge.md")
+	if err := os.WriteFile(p, []byte("  I am a focus-tool author.\n"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	got, err := NewFileSource(p).Load(context.Background(), "")
+	if err != nil {
+		t.Fatalf("Load: %v", err)
+	}
+	if got.Text != "I am a focus-tool author." {
+		t.Errorf("Text = %q (whitespace should be trimmed)", got.Text)
+	}
+	if got.Path != p {
+		t.Errorf("Path = %q, want %q", got.Path, p)
+	}
+}
+
+func TestLoadAbsentFileIsNotError(t *testing.T) {
+	p := filepath.Join(t.TempDir(), "missing.md")
+	got, err := NewFileSource(p).Load(context.Background(), "")
+	if err != nil {
+		t.Fatalf("absent file must not error: %v", err)
+	}
+	if got.Text != "" {
+		t.Errorf("Text = %q, want empty for absent file", got.Text)
+	}
+	if got.Path != p {
+		t.Errorf("Path = %q, want %q even when absent", got.Path, p)
+	}
+}
+
+func TestLoadExplicitPathBeatsDefault(t *testing.T) {
+	dir := t.TempDir()
+	def := filepath.Join(dir, "default.md")
+	other := filepath.Join(dir, "other.md")
+	if err := os.WriteFile(def, []byte("default"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(other, []byte("explicit"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	got, err := NewFileSource(def).Load(context.Background(), other)
+	if err != nil {
+		t.Fatalf("Load: %v", err)
+	}
+	if got.Text != "explicit" || got.Path != other {
+		t.Fatalf("explicit path ignored: %+v", got)
+	}
+}
+
+func TestLoadTruncatesOversize(t *testing.T) {
+	p := filepath.Join(t.TempDir(), "big.md")
+	if err := os.WriteFile(p, []byte(strings.Repeat("a", maxProfileBytes+500)), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	got, err := NewFileSource(p).Load(context.Background(), "")
+	if err != nil {
+		t.Fatalf("Load: %v", err)
+	}
+	if len(got.Text) > maxProfileBytes+len("\n…(truncated)") {
+		t.Errorf("Text not truncated: %d bytes", len(got.Text))
+	}
+	if !strings.HasSuffix(got.Text, "(truncated)") {
+		t.Errorf("missing truncation marker: %q", got.Text[len(got.Text)-20:])
+	}
+}
+
+func TestLoadWhitespaceOnlyIsAbsent(t *testing.T) {
+	p := filepath.Join(t.TempDir(), "blank.md")
+	if err := os.WriteFile(p, []byte("   \n\t\n"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	got, err := NewFileSource(p).Load(context.Background(), "")
+	if err != nil {
+		t.Fatalf("Load: %v", err)
+	}
+	if got.Text != "" {
+		t.Errorf("whitespace-only should be empty, got %q", got.Text)
+	}
+}
+
+func TestLoadReadErrorIsError(t *testing.T) {
+	dir := t.TempDir() // a directory is not readable as a file
+	if _, err := NewFileSource(dir).Load(context.Background(), ""); err == nil {
+		t.Fatal("reading a directory should error")
+	}
+}