fix(git-master): inject watermark only when enabled instead of overriding defaults

The watermark (commit footer and co-author) was inconsistently applied because:
1. The skill tool didn't receive gitMasterConfig
2. The approach was 'default ON, inject DISABLED override' which LLMs sometimes ignored

This refactors to 'inject only when enabled' approach:
- Remove hardcoded watermark section from base templates
- Dynamically inject section 5.5 based on config values
- Default is still ON (both true when no config)
- When both disabled, no injection occurs (clean prompt)

Also fixes missing config propagation to skill tool and createBuiltinAgents.

src/plugin-handlers/config-handler.ts

@@ -104,7 +104,8 @@ export function createConfigHandler(deps: ConfigHandlerDeps) {
       pluginConfig.agents,
       ctx.directory,
       config.model as string | undefined,
-      pluginConfig.categories
+      pluginConfig.categories,
+      pluginConfig.git_master
     );
 
     // Claude Code agents: Do NOT apply permission migration