Replace implicit note shorthand with explicit addnote command and split README

Two changes:

1. structured-add-commands: The implicit note shorthand (kb "text") caused
   accidental note creation from mistyped commands. Replaced with explicit
   kb addnote <text> command. Root command reverts to standard Cobra
   behaviour. Updated examples, tests, SKILL.md, and specs.

2. split-readme-developer-docs: Moved build-from-source instructions, release
   process, API reference, and ROCm migration notes from README.md into a
   new DEVELOPER.md. README now links to DEVELOPER.md for dev workflows.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

client/cmd/examples.go

@@ -11,9 +11,9 @@ var examplesCmd = &cobra.Command{
 	Short: "Show common usage examples",
 	Args:  cobra.NoArgs,
 	Run: func(cmd *cobra.Command, args []string) {
-		fmt.Print(`Quick notes:
-  kb "Remember to update DNS records"
-  kb "Server room is building 3" --tags ops
+		fmt.Print(`Add notes:
+  kb addnote "Remember to update DNS records"
+  kb addnote "Server room is building 3" --tags ops
 
 Add files:
   kb addfile report.pdf