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/knowledge.go

@@ -0,0 +1,23 @@
+// Package knowledge is the Knowledge port: it answers "who am I; what are my
+// priorities?" by loading the user's standing profile. It imports nothing from
+// the rest of the app, so it stays a leaf package.
+package knowledge
+
+import "context"
+
+// Profile is the user's standing context. Primitives only, so knowledge stays a
+// leaf package.
+type Profile struct {
+	Text string // grounding text; "" when no profile is available
+	Path string // resolved source location, for display
+}
+
+// Source answers "who am I; what are my priorities?" — the user's standing
+// profile that grounds the advisor.
+type Source interface {
+	// Load returns the user's profile. path selects an explicit location; ""
+	// means the adapter's configured default. A missing source is NOT an error:
+	// it yields an empty-Text Profile so the caller degrades to ungrounded. Only
+	// a real read failure (permissions, unreadable) returns an error.
+	Load(ctx context.Context, path string) (Profile, error)
+}