refactor(usage): remove DEFAULT_FEATURE constant and derive feature from command context

Remove the hardcoded DEFAULT_FEATURE ("cli") constant from kilo-gateway
and instead derive the feature value at runtime in opencode. When the
KILOCODE_FEATURE env var is not set, the "serve" command is now tagged
as "unknown" (to surface misconfiguration when a caller forgets to set
the env var), while all other commands default to "cli".

Author: Pedro Heyerdahl

packages/kilo-gateway/src/api/constants.ts

@@ -64,9 +64,6 @@ export const TESTER_SUPPRESS_VALUE = "SUPPRESS"
 /** Header name for feature tracking */
 export const HEADER_FEATURE = "X-KILOCODE-FEATURE"
 
-/** Default feature value (CLI usage) */
-export const DEFAULT_FEATURE = "cli"
-
 /** Environment variable name for feature override */
 export const ENV_FEATURE = "KILOCODE_FEATURE"
 // kilocode_change end