Reject single bare word as implicit note shorthand

Single unrecognized words now print an error with usage hint instead of
being submitted as a note. Prevents typos from creating junk notes.

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

openspec/specs/go-client/spec.md

@@ -70,7 +70,7 @@ The client SHALL provide a `kb search <query>` command that sends the query to t
 
 ### Requirement: Implicit note shorthand
 
-The client SHALL treat bare string arguments (with no subcommand) as an implicit note. `kb "my note"` SHALL behave identically to submitting a note via `POST /api/v1/jobs`. All persistent flags (`--format`, `--engine`, `--api-key`) and the root `--tags` flag SHALL work with the shorthand form.
+The client SHALL treat bare string arguments (with no subcommand) as an implicit note only when **more than one argument** is provided. `kb "my note"` SHALL behave identically to submitting a note via `POST /api/v1/jobs`. All persistent flags (`--format`, `--engine`, `--api-key`) and the root `--tags` flag SHALL work with the shorthand form. A single bare word SHALL be rejected with an error message.
 
 #### Scenario: Quick note via bare argument
 - **WHEN** the user runs `kb "remember to update DNS"`
@@ -92,6 +92,10 @@ The client SHALL treat bare string arguments (with no subcommand) as an implicit
 - **WHEN** the user runs `kb remember to update dns` (without quotes)
 - **THEN** the client SHALL join all arguments into a single note string and submit it
 
+#### Scenario: Single bare word rejected
+- **WHEN** the user runs `kb infow` (a single unrecognized word)
+- **THEN** the client SHALL print to stderr: `Unknown command "infow". Run 'kb --help' for available commands.` followed by a hint about note usage, and exit with a non-zero code
+
 #### Scenario: No interference with subcommands
 - **WHEN** the user runs `kb search "query"` or any other existing subcommand
 - **THEN** the client SHALL route to the subcommand as before — the implicit note shorthand SHALL NOT interfere