fix: IMP-4/5/6 — parser allowlist, delete ID resolution, consistent errors

IMP-5: Replace strings.Contains(arg, ":") heuristic with an allowlist
of recognized attribute keys (ValidAttributeKeys). Colons in task
descriptions (URLs, "Meeting: topic") are no longer misinterpreted as
modifiers. Canonical key sets live in engine/keys.go and are shared
across parseAddArgs, ParseFilter, and ParseModifier. ParseModifier now
errors on unknown keys.

IMP-4: delete command now loads the working set and resolves display IDs
via GetTaskByDisplayID, matching the pattern used by done/modify.

IMP-6: All action commands (done, delete, modify, start, stop) now
return an error on no-match (stderr, exit 1). Previously done/delete
printed to stdout and exited 0; start/stop had no check at all.

Also adds requirements and design docs for the CLI UX improvements.

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

opal-task/internal/engine/keys.go

@@ -0,0 +1,30 @@
+package engine
+
+// ValidAttributeKeys are the recognized key:value attribute names.
+// Used by parseAddArgs (cmd/add.go), ParseFilter, and ParseModifier
+// to distinguish modifiers from description text.
+var ValidAttributeKeys = map[string]bool{
+	"due":       true,
+	"priority":  true,
+	"project":   true,
+	"recur":     true,
+	"status":    true,
+	"wait":      true,
+	"scheduled": true,
+	"until":     true,
+}
+
+// DateKeys is the subset of ValidAttributeKeys that hold date values.
+// Used by Modifier.Apply and Modifier.ApplyToNew for date parsing.
+var DateKeys = map[string]bool{
+	"due":       true,
+	"scheduled": true,
+	"wait":      true,
+	"until":     true,
+}
+
+// FilterOnlyKeys are additional keys valid in filter context but not as
+// modifiers (e.g., uuid is a filter target, not something you set via modify).
+var FilterOnlyKeys = map[string]bool{
+	"uuid": true,
+}