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.go

@@ -0,0 +1,92 @@
+package knowledge
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"io/fs"
+	"os"
+	"path/filepath"
+	"strings"
+	"unicode/utf8"
+)
+
+// maxProfileBytes caps the grounding text so the coach prompt stays bounded.
+const maxProfileBytes = 6 * 1024
+
+// FileSource reads the user's profile from a single file. It is stateless: the
+// selected path is passed to Load, so the controller can repoint it at runtime
+// without a mutable field.
+type FileSource struct {
+	defaultPath string // used when Load is called with path == ""
+}
+
+// NewFileSource builds the adapter. defaultPath is used when Load receives an
+// empty path; if it too is empty, Load falls back to ~/.antidrift/knowledge.md.
+func NewFileSource(defaultPath string) *FileSource {
+	return &FileSource{defaultPath: defaultPath}
+}
+
+// Load reads the profile at path (or the default). A missing file yields an
+// empty-Text Profile and no error; only a real read failure errors. The
+// resolved absolute path is always returned for display.
+func (s *FileSource) Load(ctx context.Context, path string) (Profile, error) {
+	resolved := s.resolve(path)
+	data, err := os.ReadFile(resolved)
+	if errors.Is(err, fs.ErrNotExist) {
+		return Profile{Path: resolved}, nil
+	}
+	if err != nil {
+		return Profile{Path: resolved}, fmt.Errorf("knowledge: read %s: %w", resolved, err)
+	}
+	text := strings.TrimSpace(string(data))
+	if text == "" {
+		return Profile{Path: resolved}, nil
+	}
+	return Profile{Text: truncate(text, maxProfileBytes), Path: resolved}, nil
+}
+
+// resolve picks path, else the default, else ~/.antidrift/knowledge.md; expands
+// a leading ~; and makes the result absolute for stable display.
+func (s *FileSource) resolve(path string) string {
+	p := path
+	if p == "" {
+		p = s.defaultPath
+	}
+	if p == "" {
+		if home, err := os.UserHomeDir(); err == nil {
+			p = filepath.Join(home, ".antidrift", "knowledge.md")
+		}
+	}
+	p = expandTilde(p)
+	if abs, err := filepath.Abs(p); err == nil {
+		return abs
+	}
+	return p
+}
+
+func expandTilde(p string) string {
+	if p != "~" && !strings.HasPrefix(p, "~/") {
+		return p
+	}
+	home, err := os.UserHomeDir()
+	if err != nil {
+		return p
+	}
+	if p == "~" {
+		return home
+	}
+	return filepath.Join(home, p[2:])
+}
+
+// truncate clips s to max bytes on a UTF-8 rune boundary and appends a marker.
+func truncate(s string, max int) string {
+	if len(s) <= max {
+		return s
+	}
+	cut := max
+	for cut > 0 && !utf8.RuneStart(s[cut]) {
+		cut--
+	}
+	return s[:cut] + "\n…(truncated)"
+}